The people with knowledge of the investigation could not discuss details publicly and spoke to The Associated Press on condition of anonymity. Details of the investigation were first reported by The New York Times.
Gaetz has vehemently denied the allegations and said in a statement earlier this week: "No part of the allegations against me are true."
The Gaetz investigation, which has been going on since at least the summer of 2020, reached the highest levels of the Justice Department, and former Attorney General William Barr was briefed on the matter several times, the people said.
In the past few months, investigators have also interviewed several witnesses in the case and have been scrutinising documents, including travel and financial records, the people said.
Greenberg was the elected tax collector in Seminole County near Orlando when he resigned last June after his arrest on federal charges. Initially, those included stalking a political opponent, trafficking a minor for sex and illegally using a state database to create fake drivers licenses and other ID cards.
Since then, the case against Greenberg has ballooned to more than 30 charges, including wire fraud and other charges involving efforts to divert at least $400,000 from the tax collectors office into cryptocurrency he controlled and for his own personal use, such as the purchase of memorabilia autographed by NBA greats Michael Jordan and Kobe Bryant, federal prosecutors say. Other charges accuse him of using two companies he controlled to attempt to fraudulently obtain coronavirus relief funds.
Gaetz has said the allegations against him are part of an extortion plot by an attorney in Florida, who worked decades ago as a federal prosecutor and whom Gaetz identified as David McGee.
In response, McGee's law firm said the allegation that Gaetz made about the supposed extortion attempt was "both false and defamatory." The firm said that McGee had served in top positions at the US attorney's office in Florida about 25 years ago and that his "reputation for integrity and ethical conduct was impeccable".
The Justice Department has a separate investigation into the extortion allegations, being led out of the US attorney's office in northern Florida, according to a person familiar with the investigation who also could not discuss the matter publicly and spoke to the AP on condition of anonymity.
Gaetz has said his family has been cooperating with the FBI and his father was wearing a recording device, at the FBI's direction, "to catch these criminals." He also demanded the Justice Department release the recordings.
The Justice Department has not commented.
