US lawmaker Katie Hill is under congressional investigation amid allegations she had an affair with a Capitol Hill aide.
The House of Representatives Ethics Committee launched the inquiry after a conservative blog published naked images of the California Democrat.
The blog reported that Ms Hill, who is bisexual, had engaged in a three-way consensual romance.
She has asked police to investigate the nude photos published online.
Hours before the congressional inquiry was launched on Wednesday, Ms Hill, 32, sent a letter to supporters acknowledging a "relationship" with a female 2018 campaign staff member. She described that affair as "inappropriate".
But she made no mention of allegations published last week in conservative blog RedState that she is romantically linked with a male aide of hers in Congress.
That alleged affair, which Ms Hill has previously denied, is being scrutinised by the ethics committee as it would be a violation of House rules.
In the statement announcing the investigation, the ethics committee noted that the opening of an inquiry does not mean "that any violation has occurred".
The relationship that Ms Hill has acknowledged is outside of congressional purview because it occurred before she took office.
Ms Hill is the vice-chairwoman of the House Oversight and Reform Committee, which has been investigating President Donald Trump's financial dealings.
Angela Underwood Jacobs, a Republican candidate running against Ms Hill next year, accused the Democrat of "extremely poor judgement" and called on her to resign.
She said the relationship allegations "cross the line of ethical boundaries that members of Congress should be held to".
In her letter to constituents, Ms Hill said he was writing to "put to rest some of the gossip, innuendo and lies that have been told in recent days" as part of a "smear campaign".
She said the campaign relationship began during the final "tumultuous" years of her marriage.
"I know that even a consensual relationship with a subordinate is inappropriate, but I still allowed it to happen despite my better judgment," she continued. "For that I apologise."
Her apology comes a week after RedState published images of the nude lawmaker brushing the hair of a woman they described as a congressional staffer.
Ms Hill said in her letter: "The truth is, distributing intimate photos with the intent to publish them is a crime, and the perpetrator should be punished to the full extent of the law."
The congresswoman, who represents the 25th district in southern California, added that Capitol Police had been asked to investigate.
On Thursday, the Daily Mail published additional nude images of Ms Hill, triggering her lawyers to threaten a lawsuit under California's revenge porn law.
Ms Hill said she was in the process of divorcing her husband, Kenny Heslep, and accused him of trying to humiliate her.
She added: "I, like many women who have faced attacks like this before, am stronger than those who want me to be afraid."
Two years ago during her congressional campaign, she described Mr Heslep, whom she wed in 2010, in a social media post as "my best friend and the love of my life".