A politician who emerged as a central figure in the US Justice Department’s sex trafficking investigation into representative Matt Gaetz has pleaded guilty to six federal charges and agreed to co-operate with prosecutors as part of a plea deal.

Joel Greenberg, a long-time associate of Mr Gaetz, appeared in federal court in Orlando. He pleaded guilty to six of the nearly three dozen charges he faced, including sex trafficking of a minor, and he admitted that he had paid at least one underage girl to have sex with him and other men.

Mr Gaetz was not mentioned in the plea agreement or during the court hearing. But Greenberg’s co-operation – as a key figure in the investigation and a close ally of Mr Gaetz – may escalate the potential legal and political liability that the congressman is facing.

Federal prosecutors are examining whether Mr Gaetz and Greenberg paid underage girls or offered them gifts in exchange for sex.

Investigators have also been looking at whether Mr Gaetz and his associates tried to secure government jobs for some of the women. They are also scrutinising Mr Gaetz’s connections to the medical marijuana sector, including whether his associates sought to influence legislation Mr Gaetz sponsored.

Mr Gaetz has denied the allegations and any accusation of wrongdoing and has said repeatedly he will not resign from Congress.

A spokesman for the congressman has said Mr Gaetz “never had sex with a minor and has never paid for sex”.