Federal prosecutors are examining whether U.S. Rep. Matt Gaetz of Florida obstructed justice during a phone call he had with a witness in his sex crimes investigation.

The witness, according to two sources quoted by Politico, was one of several women who became acquainted with Gaetz via his one-time “wingman,” former Seminole County, Florida, tax collector Joel Greenberg, who pleaded guilty last month to a host of crimes, including sex trafficking a 17-year-old in 2017.

Gaetz, a close ally of former President Donald Trump, is said to be under investigation into whether he violated sex trafficking laws by having sex with an underage girl. Greenberg, who has been meeting with prosecutors for at least five months, has told investigators that Gaetz had sex with the girl and knew that she was being paid.

Greenberg has agreed to cooperate with the Justice Department’s investigations and could be a key witness if the department charges the Republican lawmaker.

Greenberg admitted he and unidentified others had paid a 17-year-old girl for sex and provided her drugs. He admitted that he “introduced the minor to other adult men, who engaged in commercial sex acts” with her, according to the documents, and that he was sometimes present. The others have not been named.

Gaetz, 38 and now in his fifth year in Congress, has been battling to preserve his political career since reports he is under federal investigation. He continues to deny the allegations and adamantly maintains he will not resign.

“First, I have never, ever paid for sex,” he wrote in an April op-ed in the Washington Examiner. “And second, I, as an adult man, have not slept with a 17-year-old.”

A Florida state lawmaker last year alleged, according to Fox News, that when Gaetz was in the Florida House of Representatives he participated in a competition with fellow male lawmakers to earn points for their sexual conquests with “aides, lobbyists and married legislators.”