The FBI is investigating possible corruption involving New Jersey Sen. Bob Menendez and Florida eye surgeon Salomon Melgen, The Miami Herald reported today.

FBI agents raided Melgen’s offices Tuesday night, but the agency would not comment on the nature of its investigation. The Herald reports today that the raid was primarily focused on potential Medicare fraud involving Melgen, but that a separate corruption investigation is also ongoing. The newspaper did not cite its sources.

Menendez's office did not immediately respond to a request for comment Thursday morning, but a spokeswoman said earlier this week that the office had not been contacted by investigators. An attorney for Melgen told the Herald, "we are confident that Dr. Melgen has acted appropriately at all times."

The most explosive allegations tied to Menendez and Melgen's friendship involve an elusive tipster who claims to have knowledge of Melgen flying Menendez to the Dominican Republic and arranging for nights with prostitutes, some of whom are alleged to be underage.

Menendez strongly denied any connection to prostitutes Wednesday, calling such accusations “false” and the product of a “right-wing blog” – the Daily Caller, which breathed life into the prostitution story just before Election Day, and then raised the issue again Jan. 24, four days before the raid broke the accusations into widespread view.

The primary, publicly-known accuser cited by the Caller has e-mailed with a Washington ethics group, reporters and the FBI but appears to have refused to speak to any of them by phone or in person, communicating only electronically.

The ethics group – Citizens for Responsibility and Ethics in Washington – cast doubt on the accuser's claims Wednesday, noting that he came forward four years after he says Menendez first visited prostitutes, and in the midst of Menendez’s re-election campaign, which the Senator won in November. CREW’s executive director, Melanie Sloan, said she was “increasingly skeptical” of the claims.

“There is something very suspicious about a source who repeatedly and for months refused to speak by phone to either us, other news outlets, or the FBI," Sloan told the Inquirer Wednesday.

E-mails purported to be between the accuser and FBI were anonymously posted late last week on a Web site devoted to the allegations against Menendez. The tipster, going by the name Peter Williams – possibly after Harrison “Pete” Williams, a former New Jersey Senator who resigned in a corruption scandal in 1982 -- appears to have been in contact with the FBI for five months, but as of late December had still refused repeated requests to meet with an FBI agent (if the e-mails are real and accurate).

Melgen is a heavy donor to Menendez and other Democrats and has flown the Senator several times on his private plane. Menendez did not repay Melgen for two of the flights until two years after the trips – he did so Jan. 4, after Republicans had raised questions about the trips.