A USA Swimming official who resigned in protest of transgender swimmer Lia Thomas told Fox News on Monday that she can't stand by and watch as girls are "thrown under the bus" by "biological" male competitors.

Cynthia Millen stepped down last week after officiating USA Swimming meets for over three decades. Her decision came days before the U.S. Paralympics Swimming National Championships in Greensboro, N.C.

"The fact is that swimming is a sport in which bodies compete against bodies. Identities do not compete against identities," Millen said in an appearance on "Tucker Carlson Tonight." "Men are different from women, men swimmers are different from women, and they will always be faster than women."

Parents who were outraged at the NCAA for allowing University of Pennsylvania transgender swimmer Lia Thomas to compete and dominate in women’s competitions wrote a letter to the college athletics’ governing body demanding a rule change.

Parents who were outraged at the NCAA for allowing University of Pennsylvania transgender swimmer Lia Thomas to compete and dominate in women’s competitions wrote a letter to the college athletics’ governing body demanding a rule change. (Penn Athletics)

In her resignation letter, Millen said that she felt University of Pennsylvania transgender athlete Lia Thomas's record-setting dominance of collegiate women's swimming was grossly unfair, writing that she can "no longer participate in a sport that allows biological men to compete against women."

Thomas previously competed as a man at Penn for two full seasons, most recently in 2019. NCAA rules dictate that transgender players must partake in one year of testosterone suppression before they can compete as women.  

SWIMING WORLD EDITOR SAYS TRANS SWIMMER'S ADVANTAGE THE SAME AS ‘DOPING’

"It’s horrible," Millen said. "The statement for women then is you do not matter, what you do is not important, and little girls are going to be thrown under the bus by all of this," Millen said, telling Fox News guest host Sean Duffy that Thomas is "going to be destroying women’s swimming."

"USA swimming recognizes that boys swim differently from girls," she explained. 

Millen detailed the different biological factors that impact a swimmer's performance, explaining that "boys will always have larger lung capacity, larger hearts, greater circulation, a bigger skeleton, and less fat."

Parents at Penn sent a letter to the NCAA demanding a rule change for transgender athletes

Parents at Penn sent a letter to the NCAA demanding a rule change for transgender athletes ( )

"While Lia Thomas is a child of God, he is a biological male who is competing against women," Millen said, "And no matter how much testosterone suppression drugs he takes, he will always be a biological male and have the advantage."

Thomas recently dominated a women’s competition at the Zippy Invitational at the University of Akron, including the 500-yard freestyle preliminaries and finals. She earned a winning time of 4:34.06 in the finals, which is a new Ivy League record.

Millen said it would be a "travesty to throw away" the legacy of female Olympic swimmers, such as Janet Evans and Jenny Thompson.

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"All these women who worked so hard before Title IX when they didn’t have the opportunities that men had. It would be such a shame, such a travesty to throw it away now. This is what will happen."