University of Pennsylvania transgender swimmer Lia Thomas set a new pool record to win the women’s 500-yard freestyle event at the Ivy League Championships – beating her nearest rival by more than seven seconds.
In a result that is certain to ignite the fierce debate in the US over whether transgender athletes should be allowed to compete against participants born as women, Thomas built a huge lead during the second half of the race as she broke Olympian Kate Ziegler's record.
Thomas has been at the center of controversy since she initially beat college records and earned a series of comfortable wins to qualify for the national championships.
Numerous bills in US states have sought to legally stop transgender athletes from competing against rivals born as women, achieving varying degrees of success amid passionate arguments and protests on both sides of the row.
Supporters of the bills believe that athletes born as men – particularly those who pass through puberty before transitioning, as Thomas has – have an unfair advantage and risk denying female-born prospects the chance to realize their full potential.
Their opponents say that outlawing trans athletes from taking part is discriminatory.
22-year-old Thomas is said to have been the top seed in the 200-yard, 500-yard and 1,650-yard freestyle events.
Yale University’s Iszac Henig, who is competing as a woman while transitioning from female to male, beat his own record to win the 50-yard freestyle on Thursday.
Henig beat Thomas in a race in January, although that heralded accusations that Thomas had not put in maximum effort on her way to an unusually slow sixth-place finish.
Former college swimmer Emily Kreps, of the Pennsylvania Family Institute, said that Thomas had posted the 17th fastest in history for women's swimming.
"My heart breaks for the destruction of women's sports," she added. "The governing sports bodies have let women down.
"ESPN commentators couldn't help themselves but admire Thomas' athletic prowess and commanding lead over the female swimmers.
"Never once did they comment that Thomas may have this advantage because Thomas is a biological male.
"They said more than once that it was 'racing for second' in a women's competition. We need bills... to pass to protect women and women's sports."
Thomas was not made available for media duties afterwards. She is said to have met eligibility rules by receiving hormone therapy for more than two years.
Governing body USA Swimming recently ruled that a three-member panel will now decide whether women who have transitioned possess an unfair advantage during female competition against biologically-born women.
The National Collegiate Athletic Association has also issued a new policy allowing each sport to set its own rules. Leaders remain under pressure to take further measures.
Penn senior Andie Myers reportedly wore a facemask with a transgender flag to show backing for Thomas. "I want everyone at this meet to know that I support her,” she told ESPN.
“She’s worked for all of this and she’s given up so much to transition and to be authentically herself. I think it’s really important and I think it’s really brave what she’s doing today.”
A Thomas supporter on social media told Kreps: "It wasn't a 'race for second place' for all the women.
"A woman named Lia Thomas won – through her skill and her perseverance.
"I'm a man who's been swimming since I was five but I couldn't have beaten any of these women. Lia Thomas is awesome. We should all be cheering her on."