OMAHA — At her first Olympic trials, Dana Vollmer was upset because she did not win. Never mind that she was 12 years old, the youngest competitor striving to qualify for the United States team in 2000. After she tied for 43rd in the 100-meter butterfly, Vollmer stomped off the pool deck scowling.

“I remember my dad reminding me of the other 54 grown women that a 12-year-old just beat,” Vollmer said with a laugh, “and that made me feel a little better.”

Vollmer, 28, is all grown up now, with a child of her own. But when she stepped to the blocks Monday for the 100-meter butterfly final at the trials, Vollmer channeled her precocious preteenage self.

“I’m kind of tapping into that little girl,” she said, “the one who doesn’t care who’s next to her, who just loves to race.”

Already at these trials, one reigning Olympic champion, Ryan Lochte, had failed to make the United States team in his gold medal event, the 400 individual medley, finishing third in Sunday’s final.

On Monday, two other Olympians, Matt Grevers and Missy Franklin, faced fierce challenges in the men’s and women’s 100-meter backstroke, which they won at the London Games. Grevers posted the third-fastest time in the semifinals while Franklin squeezed into the top eight, placing seventh, one-hundredth of a second ahead of Natalie Coughlin, a three-time Olympian.

Against that backdrop, Vollmer staked her claim to a third Olympic berth, finishing second to Kelsi Worrell, the gold medalist in the 100 butterfly at last summer’s Pan American Games. Worrell, 21, finished in 56.48 seconds, the second-fastest performance this year behind the 55.68 of Sarah Sjostrom of Sweden. Vollmer touched in 57.21 as her husband, Andy Grant, and their 15-month-old son, Arlen, looked on from the stands at CenturyLink Center.

Sixteen years ago, Vollmer’s goal was to qualify for five Olympics. “I remember when I was 12, I was like, ‘I’m going to get the Olympic rings tattoo and have an Olympic date in each circle,’” she said.

In 2004, she made the United States team that competed at the Athens Games, finishing sixth in the 200-meter freestyle and winning gold as a member of the 4x200-meter relay. In 2012, Vollmer added two more relay golds to her individual gold in the 100 butterfly.

Vollmer stopped swimming after the 2013 world championships, but she never signed the papers to opt out of the drug-testing program, an act that would have made her retirement official. “There was definitely just some gut feeling that I wasn’t ready,” Vollmer said.

She continued: “I wanted everyone to think I was done. That allowed me kind of the freedom to let go of anybody expecting me to swim and stay in the sport. Especially after having Arlen, nobody expected me to come back. Then it just felt really like taking ownership of my swimming career.”

Vollmer got the itch to return while on bed rest late in her pregnancy. She had added more than 50 pounds to her 6-foot frame, tipping the scale at 200. “It was like, ‘Whoa, cool!’” Vollmer said. “I always felt like if I wanted to, I know how to get back in shape and eat well and lose it.”

Over the next several months, her weight dropped, and her 100 butterfly times followed suit. Vollmer entered Monday’s final with the sixth-fastest 100 butterfly of the 2015-16 season.

Vollmer was not the only mother to race in the 100 butterfly. Erika Erndl, 38, who is married with a 20-month-old son, did not make the final but won her heat, which consisted of 10 swimmers and included two 15-year-olds and only two other competitors older than 19. Erndl, seventh at the halfway point, reeled in the youngsters on the final 50 meters. The way she felt in the final 25, “that’s the old me,” she said.

Erndl, who taught first grade until her son was born and is now a swim instructor, said she had “so much respect” for Vollmer. “I completely underestimated how hard it would be to take care of a little boy and swim at this level,” Erndl said.

Erndl and Vollmer are in select company this week: They are among the eight participants who competed at the 2000 trials. The others are Grevers, Coughlin, Michael Phelps, Anthony Ervin, Kim Vandenberg and Amanda Weir.

They have known each other since a conversation in the stands during the 2000 trials. “We were so very young,” Vandenberg said. “We were children.”

Four of the eight holdovers from 2000 have children or are soon to become parents. Grevers’s wife, Annie, is pregnant with their first child, a girl, due around Thanksgiving. And Phelps became a first-time father in May.

“It’s a really neat journey,” Grevers said. “The eight of us that have been around that long, we’re all great friends.”

He added, “It’s fun to kind of grow up with those people, trying to become parents. It’s crazy.”

Less than a month before the birth of his son, Boomer, Phelps ran into Vollmer after she had won the 100 butterfly at a trials tuneup outside Phoenix. “Good racing, mama,” he said, and then they briefly compared notes on what to expect when you are expecting.

“I feel a bond with Michael,” Vollmer said, “going from where we both were as young kids to the place where we are now.”