WASHINGTON — A former girlfriend of Representative Matt Gaetz testified on Wednesday before a federal grand jury in Orlando scrutinizing whether he broke sex trafficking laws, according to two people briefed on the case.

The development suggests that the Justice Department may have secured a key witness in its inquiry, which is seeking to determine whether Mr. Gaetz provided goods or payments to a 17-year-old girl in exchange for sex. Mr. Gaetz, a Republican, has represented a district in western Florida since 2017.

The witness, who testified in Federal District Court, dated Mr. Gaetz for about two years, beginning in 2017, and she remained in contact with him after their relationship ended, according to a person who knew the couple at the time.

She met Mr. Gaetz through one of his former confidants, Joel Greenberg, who pleaded guilty last year to a series of charges — including sex trafficking a minor — and is now cooperating in the investigation, the person said.

Mr. Gaetz is accused of having had sex with the same minor, a 17-year-old, during the span of his relationship with the former girlfriend. Mr. Greenberg has told others that he and Mr. Gaetz had sex with the girl and that she had claimed to be over 18, but they later learned her age.

Investigators have asked Mr. Gaetz’s former girlfriend about any sexual relationship that the minor may have had with Mr. Greenberg and Mr. Gaetz, and whether the girl received money or material goods like meals, drugs and hotel rooms in exchange.

Representatives for Mr. Gaetz did not immediately respond to requests for comment. He has repeatedly denied any wrongdoing in the case, including having sex with a minor and paying for sex.

Multiple people have told investigators that Mr. Gaetz’s former girlfriend is one of several women who met Mr. Greenberg through the website SeekingArrangement, which describes itself as a place where wealthy people find attractive companions and treat them to gifts and cash.

Mr. Greenberg then introduced these women to Mr. Gaetz, who often had sexual relationships with them, the witnesses have said.

Investigators began seeking evidence and testimony from Mr. Gaetz’s former girlfriend in late 2020, and the F.B.I. seized her phone at the end of that year, according to two people familiar with the investigation.

They also have asked her whether Mr. Gaetz sought to dissuade potential witnesses from cooperating with the investigation, according to people familiar with the inquiry.

CNN earlier reported that Mr. Gaetz’s former girlfriend was testifying before a federal grand jury.

One of the difficulties of prosecuting Mr. Gaetz is the credibility of many of the key witnesses.

Mr. Greenberg, who could face a decade in prison, has agreed to cooperate with the government in the hopes of receiving a lighter sentence.

But defense lawyers could seek to undermine Mr. Greenberg by citing his long history of fraud and corruption to raise doubts about his reliability.

After repeated delays, Mr. Greenberg is now scheduled to be sentenced in March. In a sign of the complexity of the case, two top prosecutors from the Justice Department in Washington joined the investigation last year — one with expertise in child exploitation and the other a top official in the public corruption unit.

Katie Benner reported from Washington, and Michael S. Schmidt from New York.