The freshman Democrat, who represents parts of northern Los Angeles and eastern Ventura counties, has been fighting what she calls a smear campaign that she describes as politically motivated.
She has denied reports that she had an inappropriate relationship with an aide after intimate photos appeared online. She blames the controversy on her husband, Kenny Heslep, as they go through a bitter divorce.
In a statement Tuesday, Hill denied having had an affair with her legislative director. She asked U.S. Capitol Police for an investigation into the intimate photos she said were posted online without her consent.
On Wednesday, Hill confirmed that she had an affair with a campaign staffer - a separate relationship and not the focus of the Ethics panel.
"During the final tumultuous years of my abusive marriage, I became involved in a relationship with someone on my campaign. I know that even a consensual relationship with a subordinate is inappropriate, but I still allowed it to happen despite my better judgment," Hill wrote in a statement sent to supporters Wednesday obtained by The Associated Press. "For that I apologize. I wish nothing but the best for her and hope everyone respects her privacy in this difficult time."
The House Committee on Ethics said it is looking into the allegations and issued this statement:
"The Committee is aware of public allegations that Representative Katie Hill may have engaged in a sexual relationship with an individual on her congressional staff, in violation of House Rule XXIII, clause 18(a). The Committee, pursuant to Committee Rule 18(a), has begun an investigation and will gather additional information regarding the allegations."
"The Committee notes that the mere fact that it is investigating these allegations, and publicly disclosing its review, does not itself indicate that any violation has occurred, or reflect any judgment on behalf of the Committee. No other public comment will be made on this matter except in accordance with Committee rules."
The allegations surfaced after the release of intimate photos on the conservative website RedState.org, which also alleged that Hill had more than one inappropriate relationship with staff members. The website posted a nude picture, reportedly involving her and one of her female campaign staff members.
Hill has asked Capitol Hill police to investigate the release of the photos without her consent.
"The fact is I am going through a divorce from an abusive husband who seems determined to try to humiliate me," Hill said. "I am disgusted that my opponents would seek to exploit such a private matter for political gain."
She also denied having had an affair with her legislative director.
Hill is vice chair of the House Oversight Committee and a freshman liaison to Democratic leaders.
In 2018, the Democratic freshman beat Republican incumbent Steve Knight, winning the last Republican-held U.S. House seat in Los Angeles County. She's also one of the few openly bisexual members of Congress.
The Associated Press contributed to this report.