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.
The Justice Department is investigating whether Representative Matt Gaetz, Republican of Florida, broke federal sex trafficking laws.
- Mr. Gaetz, 38, was elected to Congress in 2016 and became one of President Donald J. Trump’s most outspoken advocates. The inquiry focuses on the representative’s relationships with women recruited online for sex and whether he had sex with a 17-year-old girl.
- The investigation includes an examination of payments to women he met through Joel Greenberg — a former Florida tax collector who was indicted last year on a federal sex trafficking charge, among other offenses. Mr. Greenberg is expected to plead guilty to federal charges, and he has been talking to federal investigators since last year about the conduct of Mr. Gaetz and others, according to two people briefed on the matter.
- The representative has repeatedly dismissed the investigation as politically motivated and unfounded, defending his past relationships with women. So far, he has not been charged and the extent of his criminal exposure remains unclear. The investigation is continuing.
- Mr. Gaetz has claimed that his family is being targeted by two men trying to extort it for $25 million in exchange for making potential legal problems “go away.” The men have denied that they were trying to extort the Gaetzes.
- In the final weeks of the Trump administration, Mr. Gaetz asked the White House for a blanket pardon for any criminal conduct he had ever committed, people familiar with his request have said. Trump aides vetoed that idea, and Mr. Trump has said Mr. Gaetz never asked him directly for a pardon.
- Mr. Gaetz told The Times that he had no plans to resign from Congress. But as the investigation continues, he could face pressure either to step down or temporarily relinquish his spot on the House committee that oversees the Justice Department.
In the hopes of ingratiating himself with Mr. Gaetz and other well-connected Florida Republicans, Mr. Greenberg has told others, he hired women to have sex with him, Mr. Gaetz and others through a website that connected people who go on dates in exchange for gifts like fine dining, travel and allowances. During those interactions, Mr. Greenberg has told others, he and Mr. Gaetz had sex with the 17-year-old; she had claimed to be over 18, but they later learned her age.
Mr. Greenberg was first indicted in June on charges stemming from his attempts to undermine a political rival by spreading false rumors that the rival had engaged in sex with a student. Two months later, he was indicted on a charge of sex trafficking a child. Mr. Greenberg faced more charges in the following months, increasing his criminal exposure.
He initially wanted to fight the charges, but around the end of last year, as he confronted the possibility that he could face decades in prison if he were convicted, he began cooperating with federal investigators. In meetings with the investigators, he divulged details on the array of crimes he had committed and explained how he, Mr. Gaetz and others frequently paid women for sex.