WASHINGTON — The Justice Department has decided not to bring charges against Representative Matt Gaetz, Republican of Florida, after a lengthy sex-trafficking investigation, three people with knowledge of the decision said on Wednesday.

In 2021, federal prosecutors began examining whether Mr. Gaetz, a close ally of former President Donald J. Trump, broke federal sex-trafficking laws, focusing on his relationships with women recruited online for sex, and whether he had a sexual relationship with a 17-year-old girl.

A Justice Department spokesman declined to comment. The department’s decision was earlier reported by CNN.

A lawyer for Mr. Gaetz, Marc Mukasey, did not immediately respond to a request for comment.

Mr. Gaetz has maintained he did nothing wrong.

“I have a suspicion that someone is trying to recategorize my generosity to ex-girlfriends as something more untoward,” Mr. Gaetz told The New York Times in 2021. He said he had not had a sexual relationship with a minor and called other accusations of wrongdoing “unequivocally false.”

Scrutiny of Mr. Gaetz grew out of an investigation into a close ally of his, Joel Greenberg, a former tax collector in Seminole County, Fla., who was convicted of sex trafficking in December and sentenced to 11 years in prison.