Records filed in Palm Beach County show an Internal Revenue Service lien against Melgen of more than $11.1 million for unpaid taxes from 2006 through 2009. Prior liens for taxes from 1998 to 2002 were subsequently withdrawn, records show.
The Daily Caller, a conservative website, reported shortly before the November election that Menendez traveled on Melgen's private plane to the Dominican Republic to engage in sex with prostitutes.
Menendez's office said that any accusations of engaging with prostitutes "are manufactured by a politically motivated right-wing blog and are false."
At FBI headquarters in Washington, spokesman Jason Pack said the bureau "cannot comment on the existence or status of an investigation." Justice Department spokeswoman Tracy Schmaler also declined to comment.
On Tuesday, Menendez became chairman of the Senate Foreign Relations Committee, succeeding Sen. John Kerry, D-Mass.
Menendez's office said Melgen has been a friend and political supporter of the senator for many years and that the three trips that Menendez took have been "paid for and reported appropriately."
Menendez's office did not say whether the three trips were to the Dominican Republic or elsewhere. The statement did not say whether the trips were paid by Menendez personally or by his senatorial or his campaign accounts, nor did it specify how they were reported.
If Menendez did not pay for the trips, he would have to report them on annual financial disclosure forms as gifts. If he paid for the trips from his Senate office account, he would have to report them on Senate office forms. And if he used campaign funds, he would have had to report the expense on Federal Election Commission forms.
The Associated Press searched six years of office and travel-related expenses for Menendez's U.S. Senate office and found no reports reflecting payments to Melgen or trips aboard Melgen's plane. The AP also found no apparent reimbursement to Melgen in more than six years' worth of campaign expenses on file with the Federal Election Commission.
Melgen is involved in numerous businesses, all sharing the same address in West Palm Beach, according to records filed with the Department of State in Florida.
Late Tuesday and early Wednesday, FBI agents were seen inside the West Palm Beach building, walking its halls and standing beside shelves full of files.
The Daily Caller began publishing stories on its website about Menendez and Melgen on Nov. 1, when it reported that two women from the Dominican Republic said Menendez paid them for sex earlier in 2012. Prostitution is legal in the Caribbean nation.
Melgen is listed as having an ownership interest in DRM Med Assist, which Federal Aviation Administration records show is the owner of a CL-600 Challenger plane. Flight records for the aircraft were not immediately available.
Melgen, a registered Democrat, has made $193,350 in political contributions since 1998, including $14,200 to Menendez, according to Federal Election Commission records. Menendez was chairman of the Senate Democratic Campaign Committee, which raises money for Democratic Senate candidates, from 2009-2011.
Menendez, a lawyer, is a former mayor of Union City, N.J., and also served in the New Jersey state General Assembly and the New Jersey state Senate. He is divorced and has two children.
Melgen, 58, is a native of the Dominican Republic, where he earned his medical degree from the Universidad Nacional Pedro Henríquez Ureña in 1978. He has lived in the U.S. since at least 1980, holding an internship, residency and fellowship at hospitals in Connecticut, Missouri and Massachusetts, according to records filed with the Florida Department of Health.
Melgen has been licensed to practice in Florida since 1986 and purchased the West Palm Beach plot of land where he built his main office in 1991. Over the years, Melgen has become regarded as a top ophthalmologist, speaking at conferences and even operating on then-Gov. Lawton Chiles in 1997. The governor later appointed Melgen to a state panel on HMOs.
Calls to Melgen's offices Wednesday were forwarded to an answering service where receptionists told callers to try back Thursday. Calls to Melgen's home in North Palm Beach, which is appraised at $2.1 million, went unanswered.
On the website for his medical practice, Melgen writes: "I am always asked what sets me apart from most other doctors, and I would have to say that I do not consider myself to be a 'cookbook' physician. My patients are my number one priority, and when I am looking to treat a diagnosis I try to look at all the data at hand and extrapolate the best treatment instead of solely adhering to what the current 'standard' of treatment may be."
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