INDIANAPOLIS -- Katie Ledecky is heading to her fourth Olympics, cruising to victory in the 400-meter freestyle at the U.S. swimming trials on Saturday night, while Gretchen Walsh set a world record in the 100 butterfly.

Cheered on by a crowd of 20,689 at the home of the Indianapolis Colts, Ledecky touched the wall in 3 minutes, 58.35 seconds.

She improved on her time of 3:59.99 in the morning preliminaries and set herself up to make a run at another gold against a loaded field at the Paris Games. Australia's Ariarne Titmus is the defending Olympic champion and world-record holder (3:55.38), with Canadian phenom Summer McIntosh also in the mix.

The 27-year-old Ledecky is set to swim four events at the trials, all of them freestyle events ranging from 200 to 1,500 meters. She already has six individual gold medals -- more than any female swimmer in Olympic history.

"I'm pretty excited," Ledecky said after the victory ceremony. "I never imagined I would go to a fourth Olympic Games."

The expected second spot on the Olympic team will go to Paige Madden, the runner-up behind Ledecky at 4:02.08.

Walsh, meanwhile, was more than a half-second under world-record pace at the turn and finished strong to eclipse the mark of 55.48 set by Sweden's Sarah Sjöström at the 2016 Rio de Janeiro Olympics.

"I definitely was feeling it," said Walsh, who considers herself a stronger swimmer in the short-course pool. "I thought I was dying. I didn't know I was going that fast and, apparently, I took it out too fast."

Not to worry. Walsh held her left hand over her mouth as she looked at the scoreboard in disbelief, a "WR" beside her name.

"There has been a little bit of a buzz out there," she said. "I think going into tonight, like I knew it would take a (55) point-4 or I guess point-5, but I didn't think I was going to do it tonight. I just knew I wanted to go a fast time and now here I am -- a world-record holder."

The 21-year-old Walsh, a native of Nashville, Tennessee, who competes for the University of Virginia, still has some work to do to claim a spot on her first Olympic team.

In the final Sunday night, she will face a strong field that includes Torri Huske, Regan Smith and Claire Curzan -- all of them medalists from the Tokyo Games.

But Walsh feels she can go even faster.

"I still have room to grow in that race," she said.

Also Saturday, Aaron Shackell, a local favorite from suburban Carmel, earned his first trip to the Olympics with a victory in the men's 400 freestyle.

Shackell touched in 3:45.46 to hold off Kieran Smith, the bronze medalist in this event at the Tokyo Olympics. Smith used a strong finishing kick to claim the runner-up spot in 3:45.76, which should be enough to get him back to the Summer Games.

The top two in each individual event are expected to make the powerhouse American team.

Shackell's sister, Alex, finished second in the first heat of the women's 100 butterfly to qualify for Sunday's final.

Their father, Nick, was an All-American at Auburn who made England's 1996 Olympic team. Their mother, Ali, also was an All-American swimmer at Auburn and all three of their children are among the 1,007 swimmers who qualified for the nine-day meet in Indianapolis.

Information from The Associated Press was used in this report.