The FBI late Tuesday night raided the office of a South Florida eye doctor who allegedly was involved in providing young prostitutes to Sen. Bob Menendez — a claim the senator’s office has denied.

Fox News confirms that FBI agents removed items from Dr. Salomon Melgen’s West Palm Beach, Fla., business during a raid that lasted several hours. Agents could be seen rifling through items in the office and carrying boxes out of the building.

It is unclear whether the raid is related to anything involving Menendez. The Miami Herald reports that the investigation is believed to be focusing at least in part on Melgen’s finances. Melgen, a top contributor to Menendez’s federal campaigns, has an outstanding IRS lien of $11.1 million for taxes owed between 2006 and 2009, according to records obtained from the Palm Beach County recorder’s office, the newspaper reported.

But allegations also have surfaced that Menendez, with the help of Melgen, saw prostitutes while in the Dominican Republic. Emails posted online last week appeared to show the FBI communicating with an individual claiming to know about Menendez’s trips. That individual, who goes by the name of Peter Williams, confirmed to FoxNews.com that the email exchange “is totally authentic.”

He provided another purported email exchange from last week with a Miami-based FBI agent. In the exchange, the FBI agent asked to meet Williams “away from the Dominican Republic” to discuss the situation.

The director of a Washington watchdog group that received messages from Williams also confirmed to FoxNews.com that the group handed over the emails to the FBI last year.

But Melanie Sloan, director of Citizens for Responsibility and Ethics in Washington, raised concerns about the credibility of the source, saying she is “increasingly suspicious” about the alleged facts in the case.

“He refused to ever be contacted by us by phone,” she said. “It’s really easy to make a lot of allegations. … Who knows … if that’s even a real person.”

The Daily Caller first reported in November that Menendez, D-N.J., allegedly had sexual relations with several prostitutes at Melgen’s mansion in the Dominican Republic.

The senator’s office has rejected the charges for months. A spokeswoman for Menendez told FoxNews.com that the emails were “unsubstantiated garbage.”

Menendez, who joined the Senate in 2006, is becoming increasingly influential on Capitol Hill. He is poised to take over the Senate Foreign Relations Committee as outgoing chairman John Kerry becomes secretary of state. He also was part of a group of senators proposing a blueprint for immigration reform this week.

The FBI last week declined to comment on the release of the FBI emails.

Asked about the raid Tuesday night, the FBI Miami office would only say “we are conducting law enforcement in the general vicinity” of the office.

Representatives for Melgen and Menendez so far have not returned requests for comment Tuesday.

Fox News’ Serafin Gomez and FoxNews.com’s Judson Berger contributed to this report.