Melanie Sloan in 2015. (Congressional Quarterly/AP)

A high-profile Washington lawyer specializing in congressional ethics said Wednesday that Rep. John Conyers Jr. (D-Mich.) harassed and verbally abused her when she worked for him on Capitol Hill in the 1990s and that her repeated appeals for help to congressional leadership were ignored.

"There was nothing I could do to stop it," Melanie Sloan said in an interview. "Not going to leadership, not going to my boss, not going to a women's group, not going to a reporter. I was dismissed and told I must be mentally unstable."

Sloan, the former executive director of Citizens for Responsibility and Ethics in Washington (CREW), was hired by Conyers in 1995 as minority counsel to the House Judiciary Committee, where he served as the ranking Democrat. She held the job until 1998.

During that time, Sloan said, she witnessed and experienced behavior by Conyers similar to episodes described in claims against him that on Tuesday prompted the House Ethics Committee to open an investigation.

In addition to accusations of sexual misconduct, the claims against Conyers included "mistreatment of staff." Sloan said she did not believe she was sexually harassed by the congressman, but she said his behavior toward her was inappropriate and abusive. She said she was speaking publicly after seeing Conyers dismiss former staff members' accounts of misconduct.

Sloan said that Conyers routinely yelled at and berated her, often criticizing her appearance. On one occasion, she said, he summoned her to his Rayburn Building office, where she found him in his underwear.

"I was pretty taken aback to see my boss half-dressed," she said. "I turned on my heel and I left."

Arnold Reed, Conyers's legal counsel, denied Sloan's allegations and said Conyers will address complaints about his conduct after Thanksgiving. "Representative Conyers has never done anything inappropriate to Melanie Sloan," he said.

Sloan is the first former Conyers staff member to speak on the record about the 88-year-old congressman, the longest-serving member of the House and the top Democrat on the Judiciary Committee. She said she kept quiet about the incidents for 20 years because her earlier complaints were not taken seriously. She agreed to speak about her experience with Conyers after a Washington Post reporter contacted her.

"The reason I decided to go on the record is to make it easier for other people," she said. "People are afraid to come forward. So much about working in Washington is about loyalty, and you are supposed to shut up about these things."

Sloan said she complained repeatedly about Conyers's behavior to her supervisor and contacted a senior staff member in the office of then-Rep. Richard A. Gephardt (D-Mo.), the House minority leader at the time.

Gephardt said Wednesday that he did not recall Sloan raising concerns and was unaware of issues in Conyers's office. But he said congressional leadership needs to see through proposed legislation to change how harassment and abuse allegations are handled on Capitol Hill.

"This behavior, whenever it occurs, is reprehensible and can't be tolerated," Gephardt said.

The accusations against Conyers are surfacing as liberals and conservatives in Congress wrestle with mounting pressure to change Capitol Hill culture. At a hearing this month, female lawmakers said current members of Congress had groped and exposed themselves to female staffers. Lawmakers have introduced bills that would change the handling of sexual harassment and abuse complaints.

After leaving Capitol Hill, Sloan played a leading role in congressional ethics investigations as CREW's executive director. She wrote the 2004 complaint that led the House Ethics Committee to admonish then-Rep. Tom DeLay (R-Tex.) and a 2011 complaint against Sen. John Ensign (R-Nev.), who was forced to resign. She now is senior adviser to another Washington-based watchdog group, American Oversight.

The Conyers investigation began less than 24 hours after BuzzFeed News reported a 2015 settlement between Conyers and an unidentified former employee over claims of sexual harassment. On Tuesday morning, Conyers initially denied to the Associated Press that he had settled sexual harassment claims, but he later acknowledged the settlement.

"I expressly and vehemently denied the allegations made against me, and continue to do so," Conyers said in a statement.

In addition to the unidentified employee in the settlement, a former scheduler, Maria Reddick, filed a legal claim against Conyers in February saying that Conyers engaged in inappropriate sexual advances toward her and created a hostile work environment. Her complaint alleged that Conyers engaged in behavior such as "rubbing on her shoulders, kissing her forehead, making inappropriate comments, covering and attempting to hold her hand."

She said she requested to go through the formal counseling process run by the congressional Office of Compliance and subsequently requested mediation, the next step. It is unclear whether the claim was resolved.

Reddick, representing herself in the case, asked the judge to seal the case to protect Conyers's reputation. The judge denied her motion. Reddick voluntarily dismissed her lawsuit in March. Reddick did not return calls for comment. The lawsuit was first reported by BuzzFeed News, which did not name Reddick.

Conyers's spokeswoman said, "The former staffer voluntarily decided to drop the case."

Sloan said she feels a responsibility to speak up now because she knows that many young women on the Hill — frequently in low-level, low-paying positions — have less power than she did as a practicing attorney there.

"If that happened to me, and I'm a pretty strong person, what is happening to everyone else?" she said.

Julie Tate and Alice Crites contributed to this report.

The Washington Post is examining workplace violations on Capitol Hill and the process for reporting them. To contact a reporter, please email kimberly.kindy@washpost.com, michelle.lee@washpost.com or elise.viebeck@washpost.com.