The 2024 U.S. Olympic Swimming Team Trials presented by Lilly are underway from Lucas Oil Stadium in Indianapolis, Indiana.

The 20,000-plus fans that packed the home of the Indianapolis Colts were treated to a thrilling hour on Night 1, which saw the first U.S. Olympic swimming team spots clinched, a legend further her legacy and an eight-year-old world record fall.

Katie Ledecky lived up to her billing as the star attraction, locking up her place at a fourth career Olympic Games. However, the highlight of the night came in the semifinals of the women's 100m butterfly, where Gretchen Walsh became the first swimmer in 16 years to break a world record at U.S. Trials.

See below for full event-by-event recaps and results from Night 1 of the 2024 U.S. Olympic Swimming Team Trials.

Women's 100m butterfly semifinals

It only took until the second race of Night 1 of the 2024 U.S. Swimming Team Trials for a world record to fall.

Gretchen Walsh, in the second semifinal of the women's 100m butterfly, touched in 55.18 seconds to eclipse the previous mark of 55.48 seconds, previously set by Sweden's Sarah Sjostrom at the Rio 2016 Olympics.

It was the first time a world record fell at a U.S. Trials in 16 years.

Of course, Walsh still has work to do in Sunday night's final to ensure that she will be headed to Paris. 

Torri Huske won the first semifinal in 55.79 to secure her spot in the final as well.

1) Gretchen Walsh: 55.18 (WR) 
2) Torri Huske: 55.79 
3) Regan Smith: 55.92 
4) Alex Shackell: 56.78 
5) Claire Curzan: 57.24 
6) Emma Sticklen: 57.77 
7) Beata Nelson: 57.80 
8) Kelly Pash: 57.97

Men's 400m freestyle final

Aaron Shackell officially became the first member of the U.S. Olympic swimming team for the 2024 Paris Olympics, 28 years after his father, Nicholas Shackell, represented Great Britain at the 1996 Atlanta Games.

Shackell touched in 3:45.46 to not only win the event at Trials, but crucially, to achieve the Olympic standard time, thus making the 19-year-old eligible for Paris.

"Ever since I learned my dad was an Olympian, I've wanted to be an Olympian myself," Shackell said. "He inspired me to go faster."

Tokyo bronze medalist Kieran Smith finished just three tenths behind Shackell in second place. He is likely to head to a second consecutive Games, though that won't be known until later in the Trials due to team size constraints.

1) *Aaron Shackell: 3:45.46
2) **Kieran Smith: 3:45.76
3) David Johnston: 3:46.19
4) Bobby Finke: 3:46.27
5) Luke Whitlock: 3:46.55
6) Jake Magahey: 3:46.89
7) Daniel Matheson: 3:48.63
8) Jake Mitchell: 3:50.76
*Olympic place guaranteed
**Olympic place likely

Women's 400m freestyle final

Katie Ledecky is headed to her fourth Olympic Games, and a showdown with some of the biggest stars in the sport.

Swimming in front of the biggest crowd of her stellar career, the seven-time gold medalist won the women's 400m free in 3:58.35, a U.S. Trials record.

Ledecky can now set her sights on Australian world record holder Ariarne Titmus and Canadian rising star Summer McIntosh in what is expected to be one of the most anticipated events of the Paris Games. But first, Ledecky still has plenty of business to take care of at Trials in the 200m, 800m and 1500m freestyle events.

Paige Madden, a member of the U.S. silver medal-winning 4x200m freestyle relay team in Tokyo, cruised to the second place finish and will likely join Ledecky in Paris.

1) *Katie Ledecky: 3:58.35
2) **Paige Madden: 4:02.08
3) Jillian Cox: 4:06.89
4) Kayla Han: 4:08.21
5) Aurora Roghair: 4:08.66
6) Anna Peplowski: 4:09.20
7) Leah Smith: 4:09.34
8) Madi Mintenko: 4:12.05
*Olympic place guaranteed
**Olympic place likely

Men's 100m breaststroke semifinals

Nic Fink, world champion in 2024, has work to do in Sunday night's final if he's going to make it to Paris in the 100m breaststroke. Fink, swimming in the second semifinal, finished behind former Michigan Wolverine Charlie Swanson, who paced the entire field at 59.34 seconds.

The second-place qualifier, Josh Matheny, won the first semifinal in 59.42 seconds to earn a middle lane in the final.

Michael Andrew, who finished fourth in this event at the Tokyo Olympics, slotted in sixth, 0.31-seconds off the pace set by Swanson.

1) Charlie Swanson: 59.34 
2) Josh Matheny: 59.42 
3) Nic Fink: 59.46 
4) Liam Bell: 59.57 
5) Jake Foster: 59.63 
6) Michael Andrew: 59.65 
7) Noah Nichols: 59.76 
8) Luke Rodarte: 59.78