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An unnamed ex-girlfriend of Rep. Matt Gaetz, R-Fla., was granted immunity by prosecutors before her testimony in front of a federal grand jury last week as part of a sex trafficking investigation of the lawmaker, according to a report.

Rep. Matt Gaetz. (Photo by Anna Moneymaker/Getty Images)

ORGY, UNDERAGE GIRLS, SEX GAMES AND EXTORTION: INSIDE THE ALLEGATIONS SURROUND REP. MATT GAETZ

"This may be a willing participant who has a smart lawyer who sought an immunity deal from the government," CBS News legal analyst Rikki Kleiman said in a report by the network Monday. "The government does not give immunity blindly, they know what they're getting in exchange."

Investigators are trying to determine whether Gaetz violated sex trafficking laws or obstructed justice, with the woman being seen as a potential key witness in the case. One source told CBS News that she has information related to both allegations against the congressman, with investigators looking into whether Gaetz had a phone call with the ex-girlfriend and another woman who was already a witness in the probe.

Rep. Matt Gaetz

Rep. Matt Gaetz, R-Fla. (AP Photo/Marta Lavandier)

Gaetz has denied all wrongdoing in the case, saying that he has never paid for sex or had sexual relations with an underage female. An attorney for Gaetz told CBS News last week that "we have seen no credible basis for a charge against Congressman Gaetz. We remain steadfast in our commitment to challenge any allegations with the facts and law."

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Rep. Matt Gaetz

Rep. Matt Gaetz, R-Fla. (AP Photo/Phelan M. Ebenhack, File)

Part of the investigation circles around an alleged 2018 trip Gaetz took to the Bahamas with the ex-girlfriend, another woman and a third 18-year-old woman with whom Gaetz was allegedly in a sexual relationship at the time, with investigators trying to determine whether the sexual relationship started before the woman turned 18.

Investigators are also attempting to find out if any of the women were paid to have sex with the Florida lawmaker or were illegally trafficked across state and international borders to have sex with Gaetz.