Police search for suspects in Watertown, Mass. (Mario Tama/Getty Images)

BOSTON—A late-night police chase and shootout has ended with one marathon bombing suspect dead and another on the run, Boston Police commissioner Ed Davis said early Friday morning.

Federal agents swarmed neighboring Watertown after local police were involved in a car chase and shootout with at least one of the suspects. During the pursuit, officers could be heard on police radio traffic describing the suspects as having grenades and other explosives.

At approximately 10:30 p.m. Thursday, a robbery was reported at a 7-Eleven in Cambridge, Davis said. An MIT officer responding to the robbery was shot, and later pronounced dead. The suspects fled in a stolen Mercedes-Benz. Watertown Police spotted car and shots were fired between police and the suspects.

One suspect was shot by police and later died, but the second fled on foot, and a tense manhunt ensued.

"We believe this to be a terrorist," Davis told reporters at a hastily arranged press conference in Watertown. "We believe this to be a man who has come here to kill people. We need to get him into custody."

An MTA officer was seriously wounded during the exchange of gunfire, Davis said.

The FBI has yet to publicly confirm a connection between the events in Watertown and the twin explosions that killed 3 people and injured 170 others at the Boston Marathon on Monday. But according to a radio alert sent issued to fellow officers, the suspect who remained at large was the "one in the white hat" seen in the photos released by the bureau on Thursday.

The suspect was described as a "white male with dark complexion or a Middle Eastern male with thick curly hair wearing a charcoal gray hooded sweatshirt ... possibly with an assault rifle and explosives." Police in Watertown, Newton, Brighton and Cambridge were put on high alert as the suspect was said to be armed with a "long gun."

"We are aware of the law enforcement activity in the greater Boston area," Boston FBI spokesman Greg Comcowich said in a statement to Yahoo News. "The situation is ongoing. We are working with local authorities to determine what happened."

[Related: FBI releases photos of suspects in Boston Marathon bombings]

Worried residents in Watertown, a suburb about 10 miles from downtown Boston, were ordered to stay indoors and turn off their cell phones out of fear that they could trigger improvised explosive devices.

"Suspect 2" (FBI.gov)

Dozens of police officers, many of them off-duty, searched backyards in search of the second suspect, and a police perimeter of several blocks was established. K9 units and SWAT teams searched homes on Spruce Street as officers searched an SUV that the suspects had abandoned. Multiple devices were left in the road and two handguns were recovered, according to police scanners.

The Watertown shootout occurred after a gunfight erupted at the Massachusetts Institute of Technology late Thursday. A police officer was shot and pronounced dead. The campus was placed on lockdown for several hours, and students were told to remain indoors.

Shortly before 2 a.m. Friday, MIT issued a statement on its website saying that the suspect "in this evening's shooting is no longer on campus. It is now safe to resume normal activities. Please remain vigilant in the coming hours."

At approximately 3:30 a.m., Massachusetts State Police issued a plea on Twitter for residents of Watertown to lock their doors and not open them for anyone as they searched backyards and exteriors of houses there.

"Residents in and around Watertown should stay in their residences," the alert read. "Do NOT answer door unless it is an identified police officer."

WASHINGTON (AP) — Four months ago, President Barack Obamapromised a grieving nation he would do everything in his power to change gun laws after 26 students and staff were shot to death at Sandy Hook Elementary School. Turns out his power and the impassioned pleas of devastated families were no match for the force of gun rights advocates in Congress and across the nation.

The National Rifle Association and its energized supporters overcame national outrage over the deaths of innocent first graders. The Senate rejected expanded background checks for gun buyers in the face of strong public support for the change, pleas from a former congresswoman still healing from bullet wounds and a campaign bankrolled by a billionaire mayor. Foes of new controls were stronger than Obama's moral indignation from the president's "bully pulpit" and his political machine that won two elections but couldn't translate its grass-roots power to win the gun vote.

Obama, angry and defiant over the defeat, is vowing to fight on. And the NRA says it is taking him seriously. "We are prepared for a very long war and a very expensive war," association spokesman Andrew Arulanandam said Thursday.

The NRA's success is built on the passion of gun advocates, activists on both side of the debate agree. That's how they were able to defeat expanded background checks despite polling that shows up to 90 percent of Americans support the idea.

"You know what I hear from the members of Congress?" said Vice President Joe Biden. "I just met with one. He says, 'Well that may be true, Joe, but that 10 percent who doesn't agree, they are going to show up. They're going to show up and vote. And that 90 percent thinks it's a good idea, but they're not going to vote for me or against me because of how I vote on this,'" Biden said during a Google Plus online chat Wednesday.

Arulanandam said he refers to NRA members as "super volunteers" who work on political campaigns and get to know lawmakers personally so their voices are even more powerful in the debate. A Washington Post-ABC News poll taken last week shows they are more likely to speak up: 20 percent of gun owners and 14 percent of people who live with a gun owner said they contacted a public official on gun control, compared to 10 percent of adults with no gun in their home.

The changes at the heart of the gun control bill failed to get the 60 votes needed in the Senate on Wednesday. On the background-checks issue, four Democrats voted against it. They all come from states Obama lost last year. Three of the four face tough re-election fights next year.

Arulanandam rejected Obama's contention that a wide majority of NRA households actually supported the defeated legislation. "Then who was lighting up phone lines and going to town hall meetings?" he asked.

The background-check proposal was co-authored by Sen. Joe Manchin, D-W.Va., who won re-election after running an ad in which he fired a rifle and boasted of his NRA endorsement. At a breakfast sponsored by The Wall Street Journal, he predicted the legislation backed by Obama would have passed easily if the NRA hadn't threatened to use senators' votes to determine whom it would support in next year's midterm elections.

Manchin also said background checks would have been approved if the Senate had moved more quickly, when the nation's heartache over the Newtown, Conn., school shootings was still in focus. A recent Associated Press-GfK poll found that 49 percent of Americans back stricter gun laws, but that's down from 58 percent in January.

"If we'd have gone to a bill like this immediately, boom," Manchin said at the breakfast, predicting it would have gotten 65 to 70 votes. "You seize the moment."

Manchin also blamed a broader liberal agenda in Washington with making passage difficult. He said lawmakers shifting their positions on gay rights and immigration found it hard to also vote for gun control. He said constituents would ask lawmakers who made all those changes, "Are you still the same person that we sent?"

The NRA also benefits from electoral dynamics, with a group of moderate Democrats facing re-election in rural states, where residents are far more likely to live in a home with a gun. The AP-GfK poll found most urban and suburban residents — 56 percent and 52 percent, respectively — say they think gun laws should be made more strict, compared with 41 percent of rural residents.

Congressional officials said that in the run-up to Wednesday's vote, the bill's supporters tried to gain backing for expanded background checks by creating an exemption for sales that occurred in remote areas, possibly 50 miles from the nearest federally licensed gun store. The aim was to win the votes of Democratic holdouts Mark Begich of Alaska and Heidi Heitkamp of North Dakota and at least one Republican, but in the end, the effort failed.

Senate Majority Leader Harry Reid on Thursday pulled the bill before a final vote and said he would bring it back again after gun control activists have more time to make their voices heard.

"I've spoken with the president. He and I agree that the best way to keep working toward passing a background-check bill is to hit a pause and freeze the background-check bill where it is," Reid said.

Gun control supporters say they hope the defeat will energize their more-silent majority to become an increasingly powerful counterpoint to the NRA. Dan Gross, president of the Brady Campaign to Prevent Gun Violence, said chairwoman Sarah Brady reminded him after the vote that "sometimes it takes a good defeat."

She would know. After her husband, former White House press secretary Jim Brady, was partially paralyzed in the shooting of President Ronald Reagan, it took repeated tries over six years to pass a bill named after him that instituted background checks.

"We have to prove to them that this is an issue about which the overwhelming majority of the public agrees and is passionate enough to hold them accountable," Gross said in an interview. "We have to prove to them it's safe to do the right thing, and unsafe for them to do the wrong thing."

New York City Mayor Michael Bloomberg, a media executive who has financed ads aimed at electing lawmakers who support gun control, said Thursday he would work to defeat senators who voted against background checks. A fundraising email to fund a similar effort went out from Americans For Responsible Solutions, the group founded by injured former Rep. Gabrielle Giffords, D-Ariz., and her husband, Mark Kelly.

Asked which senators would be targeted for their opposition, Kelly, a former astronaut and Navy pilot said: "It's a target-rich environment, as we would say in the military."

The families of some of the Sandy Hook victims came as late entrants attempting to counter the influence of gun-rights activists on the debate. Over five days in Washington during the past two weeks, they met with 35 senators to ask for their support in memory of their children. They were credited with helping stop a block of debate on the bill, but even their emotional pleas were not enough to win passage.

Some say they, too, won't give up.

This week, for gun control advocates nurturing fading hopes of Senate passage of expanded background checks, the breaking point came Tuesday and early Wednesday, when a pair of GOP senators announced they'd oppose the effort.

Gun control supporters believed that Republican Sens. Dean Heller of Nevada and Kelly Ayotte of New Hampshire were still possible votes for the measure. But Heller said Tuesday he'd oppose the compromise — which had been worked out by Manchin and Pennsylvania Republican Pat Toomey — and Ayotte said the same Wednesday morning.

They would have been the 56th and 57th votes. Had they agreed, gun control supporters say they might have had a chance to persuade three of the four Democratic senators who ended up voting "no" to instead push the measure over the top by getting to the needed 60 votes,

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Associated Press writers Alan Fram and David Espo in Washington, Karen Matthews in New York and Director of Polling Jennifer Agiesta contributed to this report.

This undated photo obtained from the facebook page of Paul Kevin Curtis, shows, according to neighbors, Paul Kevin Curtis, 45. Curtis was arrested Wednesday, April 17, 2013, at his home in Corinth, near the Tennessee state line. He is accused of mailing letters with suspected ricin to to national leaders. (AP Photo)

OXFORD, Miss. (AP) — A Mississippi man charged with mailing ricin-tainted letters to national leaders wrote in online postings that he had uncovered a conspiracy to sell human body parts on the black market, and on Thursday his attorney said he was surprised by his arrest and maintains he is innocent.

Paul Kevin Curtis, 45, wore shackles and a Johnny Cash T-shirt Thursday in a federal courtroom. His handcuffs were taken off for the brief hearing, and he said little. He faces two charges on accusations of threatening President Barack Obama and others. If convicted, he could face up to 15 years in prison.

He did not enter a plea on the two charges. The judge said a preliminary hearing and a detention hearing are scheduled for 3 p.m. Friday.

Attorney Christi R. McCoy said Curtis "maintains 100 percent that he did not do this."

"I know Kevin, I know his family," she said. "This is a huge shock."

McCoy said she has not yet decided whether to seek a hearing to determine whether Curtis is mentally competent to stand trial.

Curtis, who was arrested Wednesday at his home in Corinth, near the Tennessee state line, was being held in the Lafayette County jail in Oxford, Miss.

An FBI affidavit says Curtis sent three letters with ricin to Obama, U.S. Sen. Roger Wicker and a Mississippi judge. The letters read:

"No one wanted to listen to me before. There are still 'Missing Pieces.' Maybe I have your attention now even if that means someone must die. This must stop. To see a wrong and not expose it, is to become a silent partner to its continuance. I am KC and I approve this message."

The affidavit says Curtis had sent letters to Wicker's office several times before with the message, "this is Kevin Curtis and I approve this message."

In several letters to Wicker and other officials, Curtis said he was writing a novel about black market body parts called "Missing Pieces."

Curtis also had posted language similar to the letters on his Facebook page, the affidavit says.

The documents indicate Curtis had been distrustful of the government for years. In 2007, Curtis' ex-wife called police in Booneville, Miss., to report that her husband was extremely delusional, anti-government and felt the government was spying on him with drones.

Curtis was arrested Wednesday at his home in Corinth, near the Tennessee state line. He was being held in the Lafayette County jail in Oxford, Miss.

Curtis had been living in Corinth, a city of about 14,000 in extreme northeastern Mississippi, since December, but local police had not had any contact with him before his arrest, Corinth Police Department Capt. Ralph Dance told The Associated Press on Thursday.

Ricky Curtis, who said he was Kevin Curtis' cousin, described his cousin as a "super entertainer" who impersonated Elvis and numerous other singers.

Wicker said Thursday in Washington that he had met Curtis when he was working as Elvis at a party Wicker and his wife helped throw for an engaged couple.

Wicker called him "quite entertaining" but said: "My impression is that since that time he's had mental issues and perhaps is not as stable as he was back then."

Wicker's spokesman, Ryan Annison, said the party occurred about 10 years ago.

Police maintained a perimeter Thursday around Curtis' home. Four men who appeared to be investigators were in the neighborhood to speak to neighbors. There didn't appear to be any hazardous-material crews, and no neighbors were evacuated.

The FBI Thursday that the letters to both Wicker and Obama contained ricin.

Ricin is derived from the castor plant that makes castor oil. There is no antidote, and it's deadliest when inhaled. The material sent to Wicker was not weaponized, Gainer said.

An FBI intelligence bulletin obtained by The Associated Press said the two letters were postmarked Memphis, Tenn.

A Mississippi state lawmaker, Democratic Rep. Steve Holland of Plantersville, said his 80-year-old mother, Lee County Justice Court Judge Sadie Holland, received a threatening letter April 10 with a substance that has been sent to a lab for testing. He said this letter was also signed "K.C."

"Like any country woman, she did a smell test," Steve Holland said. "She said, 'It sort of burned my nose a little bit.'"

He said once she read the letter, she immediately called the local sheriff.

David Daniels, a 63-year-old attorney in Tupelo, said he has had a restraining order against Curtis since 2002. He said Curtis threatened him after a rehearsal for an Elvis impersonators' show that Daniels was hired to help organize.

He filed a simple assault charge against Curtis, and Sadie Holland was the judge in that case. She sentenced him to six months in jail, Daniels said, though he served only a few days.

Sadie Holland has been "sequestered by the FBI" and told not to talk to anybody for now, and is undergoing medical tests, her son said.

Ricky Curtis said his family was shocked by news of the arrest. He said his cousin had written about problems he had with a cleaning business and that he felt the government had not treated him well, but he said nobody in the family would have expected this. He said the writings were titled, "Missing Pieces."

"I don't think anybody had a clue that this kind of stuff was weighing on his mind," Ricky Curtis said in a telephone interview.

A MySpace page for a cleaning company called The Cleaning Crew confirms that they "do windows" and has a profile photo of "Kevin Curtis, Master of Impressions." A YouTube channel under the name of Kevin Curtis has dozens of videos of him performing as different famous musicians, including Elvis Presley, Buddy Holly and Kid Rock.

Multiple online posts on various websites under the name Kevin Curtis refer to the conspiracy he claimed to uncover when working at a local hospital from 1998 to 2000. And on Thursday, his attorney said he had been an activist in the field of organ and tissue donation that stemmed from his work at that hospital.

The author wrote that the conspiracy began when he "discovered a refrigerator full of dismembered body parts & organs wrapped in plastic in the morgue of the largest non-metropolitan health care organization in the United States of America."

That hospital, North Mississippi Medical Center, issued a statement Thursday saying Curtis was fired in 2000. It did not give a reason for his firing but said it was not because of allegations he made against the hospital.

The hospital said it works with an agency that specializes in harvesting organs and tissue from donors, and then immediately transports those organs for donation. The hospital said it does not receive payment for the donated organs.

Curtis wrote that he was trying to "expose various parties within the government, FBI, police departments" for what he believed was "a conspiracy to ruin my reputation in the community as well as an ongoing effort to break down the foundation I worked more than 20 years to build in the country music scene."

In one post, Curtis said he sent letters to Wicker and other politicians.

"I never heard a word from anyone. I even ran into Roger Wicker several different times while performing at special banquets and fundraisers in northeast, Mississippi but he seemed very nervous while speaking with me and would make a fast exit to the door when I engaged in conversation..."

Jim Waide, an attorney in Tupelo, Miss., said he was working with Curtis' family Thursday to put together a statement about the man. Waide said the family told him Curtis has been diagnosed as bipolar and was put on medication about three years ago. "It's been a real problem to keep him on his medication," Waide said in a phone interview from Tupelo.

"He has a long history of mental illness," Waide said. "When he is on his medication, he is terrific, he's nice, he's functional. When he's off his medication, that's when there's a problem."

Waide represented Curtis in a federal lawsuit he filed in August 2000 against North Mississippi Medical Center in Tupelo. Curtis claimed employment discrimination. A judge dismissed the case in July 2001. Records show it was "dismissed for failure to prosecute."

Court records show Waide got a judge's permission to withdraw as Curtis' attorney in January 2001. Waide said he withdrew from the case because Curtis didn't trust him.

"He thought I was conspiring against him," Waide said. "He thinks everybody is out to get him."

The FBI said there was no indication of a connection between the letters and the Monday bombing in Boston that killed three people and injured more than 170. The letters to Obama and Wicker were postmarked April 8, before the marathon.

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Associated Press writers contributing to this report from Washington were: Eileen Sullivan, Laurie Kellman, Donna Cassata, Henry Jackson and Eric Tucker. AP news researcher Monika Mathur contributed from New York. AP writer Emily Wagster Pettus contributed from Jackson, Miss.