LAHORE, Pakistan (AP) — Authorities are investigating cough syrup believed to have killed 33 people in eastern Pakistan in the past three days, a government official said Saturday, the second time in recent months that suspect medicine is thought to have caused multiple deaths.

Also Saturday, an explosion ripped through a passenger bus while it was at a terminal in the southern city of Karachi, killing four people and wounding 40 others, police and hospital officials said. It's unclear if the blast was caused by a bomb or a gas cylinder exploding.

The deaths from the cough syrup occurred in Gujranwala and villages surrounding the city, saidAbdul Jabbar Shaheen, the top administrative official in Gujranwala. Another 54 people are being treated at hospitals in the city who are also believed to have consumed the syrup. Those involved are thought to be laborers or drug addicts who drank the syrup to get high, said Shaheen.

Chemical samples collected from the victims' stomachs contained dextromethorphan, a synthetic morphine derivative used in cough syrup that can have mind-altering effects if consumed in large quantities, said Shaheen. It is being investigated whether the people affected by the syrup in Gujranwala drank too much of it, or whether there was a problem with the medicine itself, he said.

Twenty-three people died in the nearby city of Lahore in November after drinking bad cough syrup sold under the brand name Tyno. They were also described at the time as people who consumed the drug to get high.

Shaheen said the cough syrup involved in the incidents in and around Gujranwala was not sold under a single brand. He said there were some people in the city involved in the business of making cough syrup specifically to sell to drug addicts, and officials were trying to arrest the culprits.

Officials temporarily closed one Lahore-based pharmaceutical company whose cough syrup was found in the possession of some of those affected in Gujranwala and were investigating whether it caused any of the deaths, said Shaheen.

The blast that ripped through the bus in Karachi on Saturday set the vehicle on fire and reduced it to little more than a charred skeleton. Police were trying to determine whether the explosion was caused by a bomb or a gas cylinder, said police spokesman Imran Shaukat. Many buses in Pakistan run on natural gas.

The explosion killed at least four people and wounded 40 others, some of whom were in critical condition, said Seemi Jamali, a doctor at the hospital in Karachi where they were being treated.

Karachi has a long history of political, ethnic and sectarian violence. It is also believed to be home to many Taliban militants who have fled U.S. drone attacks and Pakistani army operations in the country's northwest.

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Associated Press writer Adil Jawad contributed to this report from Karachi, Pakistan.

MAIDUGURI, Nigeria (AP) — Gunmen suspected to belong to a radical Islamist sect attacked a village in northeast Nigeria, tying up men, women and children before slitting their throats, killing at least 15 in the troubled region's latest attack, witnesses said Saturday.

The assault happened early Friday morning in the village of Musari on the outskirts of Maiduguri, the city where the sect known as Boko Haram first launched its guerrilla campaign of shootings and car bombings against Nigeria's weak central government. The gunmen shouted religious slogans and later ordered those there to be gathered up into a group, said Mshelia Inusa, a primary school teacher in the village.

"We heard some people chanting, 'God is great, God is great' amid sounds of banging on doors of houses at about 1 a.m.," the teacher said. "A voice was heard ordering people to be slaughtered and also voices of children were heard screaming."

Inusa said he and others later saw corpses with their hands tied behind their backs and their throats cut.

Later Friday morning, an ambulance arrived at the State Specialists Hospital in Maiduguri, accompanied by a group of military vehicles, a security guard said. Agitated soldiers ordered people away, but the guard said he counted at least 15 bodies being brought into the facility's morgue.

The guard spoke on condition of anonymity out of fears of angering either the military or the sect.

Lt. Col. Sagir Musa, a military spokesman, later issued a statement saying only five people had been killed in the village during the attack. However, military and police officials routinely downplay casualty figures because they are under increasing pressure from their superiors to minimize the perceived effects of the ongoing attacks by Boko Haram.

Boko Haram, which speaks to journalists through conference calls at times of its choosing, could not be immediately reached for comment Saturday. However, the attack mirrored other assaults carried out by the group as it expands its operations outside of cities in the northeast into rural towns and villages, where the security presence remains light and contact with the outside world remains difficult as the sect has destroyed a number of mobile phone towers recently.

The sect, whose name means "Western education is sacrilege" in the Hausa language of Nigeria's Muslim north, wants the nation to enact strict Shariah law and release its imprisoned members. Despite a heavy military and police presence, the sect's adherents have continued to launch frequent attacks.

More than 780 people have been killed in Boko Haram attacks so far this year, according to an Associated Press count, making 2012 the worst year of violence attributed to the group. Boko Haram also has loose connections with al-Qaida in the Islamic Maghreb and Somalia's al-Shabab, according to Western military officials and diplomats.

Suspected Boko Haram gunmen also attacked another village Friday in Adamawa state on its border with neighboring Cameroon. Witnesses said that attack focused on the town of Maiha, where gunmen also shouted praises to God while setting fire to government buildings, a school and a prison. At least 35 prisoners were released from the prison in the attack, though 11 had been recaptured, police spokesman Mohammed Ibrahim said Saturday.

Ibrahim said a civilian and a police officer were killed during the fighting.

And violence continued around the central Nigerian city of Jos, where ethnic, religious and political rivalries have caused mass killings in recent years. Authorities said at least seven had been killed in recent days around Christian villages in the rural plateau. Police said they were investigating the attacks.

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Associated Press writers Ibrahim Abdul in Yola, Nigeria, and Ahmed Saka in Jos, Nigeria, contributed to this report.