The New York Democratic congresswoman Alexandria Ocasio-Cortez is under investigation by the House of Representatives' ethics committee, the leaders of the panel said.
The Democratic acting chair, Susan Wild of Pennsylvania, and acting ranking member, Michael Guest, a Mississippi Republican, released a statement on Wednesday.
They said: "The matter regarding Representative Alexandria Ocasio-Cortez … was transmitted to the committee by the Office of Congressional Ethics (OCE) on 23 June."
The subject of the investigation was not revealed.
The committee said: "The mere fact of a referral or an extension, and the mandatory disclosure of such an extension and the name of the subject of the matter, does not itself indicate that any violation has occurred, or reflect any judgment on behalf of the committee."
A spokesperson for Ocasio-Cortez said: "The congresswoman has always taken ethics incredibly seriously, refusing any donations from lobbyists, corporations, or other special interests. We are confident that this matter will be dismissed."
The House ethics committee said it would announce its "course of action" after the new Congress convenes in January.
Ocasio-Cortez won her seat in Congress in 2018, after a shock primary victory over Joe Crowley, a senior House Democrat. She has since emerged as a leading figure among progressives, widely known as AOC and the target of rightwing invective and harassment.
Ocasio-Cortez made a splash at the $35,000-a-ticket New York society event, wearing a dress emblazoned with the slogan "Tax the Rich". A spokesperson said: "She was invited as a guest of the Met. She also did not get to keep the dress."
In 2019, in a slightly bizarre twist, it was reported that Donald Trump had become "enamored" and "starstruck" by a politician half his age and his ideological opposite, and had compared her to a historical figure made famous in America at least by a Broadway musical.
"I called her Eva Perón," Trump said, according to the book American Carnage by Tim Alberta. "I said, 'That's Eva Perón. That's Evita."
Perón, an actor married to the Argentinian president Juan Perón, championed working class and female voters but died of cancer in 1952, aged 33.
Outside Argentina she is largely known through Evita, a musical by Andrew Lloyd Webber and Tim Rice which premiered in London in 1978 and on Broadway in 1979 and which Trump has said is his favourite show, having seen it six times.
Ocasio-Cortez responded: "I know that, like every woman of the people, I have more strength than I appear to have."