A former confidant of Representative Matt Gaetz, Republican of Florida, admitted in court papers on Friday to an array of federal crimes — including sex trafficking of a minor — and agreed to cooperate with the Justice Department’s investigations, handing prosecutors a potential key witness as they decide whether to charge Mr. Gaetz.
Joel Greenberg, who was a tax collector in the Orlando area until he was indicted last year, did not implicate Mr. Gaetz by name in papers filed by prosecutors in Federal District Court in Orlando.
But Mr. Greenberg admitted that he and unidentified others had paid a 17-year-old girl for sex and that he had provided her with 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 were not named.
Prosecutors revealed in the documents that they have evidence they say corroborates Mr. Greenberg’s admissions — including a series of communications and transactions Mr. Greenberg had with the girl, and a list of dates of their sexual encounters. The inclusion of that material appeared designed to bolster the credibility of Mr. Greenberg as a witness whose truthfulness would likely be challenged by anyone who is charged based on anything he tells prosecutors.
Mr. Gaetz, a close ally of former President Donald J. Trump, is said to be under investigation into whether he violated sex trafficking laws by having sex with the same girl. Mr. Greenberg, who has been meeting with prosecutors for at least five months, has told investigators that Mr. Gaetz had sex with the girl and knew that she was being paid, according to a person briefed on the inquiry.
Mr. Greenberg also admitted that he had stolen money from local taxpayers, committed identity theft and defrauded the federal government.
As the inquiry ensnared Mr. Gaetz and other influential Florida Republicans and burst into national news in recent weeks, reports have portrayed them as a freewheeling group that frequented parties, sometimes took the mood-altering drug Ecstasy and, in some cases, paid women they had sex with.
Mr. Gaetz has denied paying for sex and said that his generosity toward former girlfriends was being misconstrued.
Although Mr. Greenberg might have deep knowledge about the activities of Mr. Gaetz and others, using him as a witness could be complicated for prosecutors because he has a history of lying and has now pleaded guilty to a range of crimes, including some after he was initially charged by prosecutors last year.
Defense lawyers often attack the credibility of such cooperators when the government uses them as witnesses at trials, sometimes arguing that they are simply testifying to what the government wants in exchange for leniency.
Mr. Greenberg, 37, is facing 12 years in prison. But if his cooperation results in the prosecutions of others, the Justice Department can file a motion with the judge overseeing his case to ask for a shorter sentence. According to court documents filed on Friday, Mr. Greenberg could face perjury charges if he misleads investigators.
The plea by Mr. Greenberg is the latest chapter in his friendship with Mr. Gaetz, which began in Republican political circles in Florida around the time Mr. Trump was elected president in 2016. Mr. Trump’s candidacy attracted many inexperienced politicians who were granted access, including Mr. Gaetz and Mr. Greenberg.
Mr. Gaetz, the son of a former Florida State Senate president, was looking to develop a following in the state and nationally. Mr. Greenberg, the son of a wealthy dentist, had struggled with addiction issues but had defeated the longtime tax collector in Seminole County, Fla., in a primary to catapult his political career.
