Joel Greenberg, a former associate of Rep. Matt Gaetz (R-Fla.), pleaded guilty on Monday to six criminal charges, including sex trafficking a 17-year-old girl and wire fraud, the Washington Post reports.

Why it matters: The Justice Department is investigating Gaetz "over whether he had a sexual relationship with a 17-year-old," per the New York Times. Greenberg, a former Florida county tax collector, could be a key witness as federal prosecutors decide whether to charge Gaetz.

• Investigators found evidence "potentially implicating Gaetz in sex trafficking" after probing Greenberg’s conduct, the Post reports, citing people familiar with the matter.
• "The allegations against me are as searing as they are false," Gaetz told Axios' Jonathan Swan in March. The Florida congressman and Trump loyalist has not been charged.

What's next: Greenberg has agreed to cooperate in the federal investigation in exchange for prosecutors dropping the other 27 counts he faced.