Story highlights Federal agents raid the office of a Florida doctor who has given to Democratic causes

A past recipient is Sen. Menendez, who flew on the doctor's plane to the Caribbean

Dr. Salomon Melgen has a stake in a port security company involved in work in the Dominican Republic

Menendez has touted better port security in Dominican Republic, but didn't mention that firm

Controversy continues to swirl around New Jersey Sen. Robert Menendez -- including his relationship with a generous Democratic donor whose plane he took to the Dominican Republic and who has business ties in that Caribbean nation.

Menendez first came under scrutiny shortly before Election Day last fall, when a conservative online publication cited three unidentified sources who claimed the senator had flown on private planes to the Caribbean and during the trip had sex parties with prostitutes

Questions revolve around his relationship with Salomon Melgen, the Florida man who owned a plane Menendez admits having flown three times -- once on official Senate business, and twice for personal reasons -- to the Dominican Republic in 2010.

Melgen, his wife, Flor, and his daughter, Melissa, have been generous donors to Menendez, his fellow Democrats and related causes in recent years.

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Since 1998, the three South Florida residents have given at least $360,000 to candidates and groups such as the Democratic National Committee and the New Jersey Democratic State Committee, according to data on the Federal Election Commission's website

This does not include the $600,000 that Melgen's then company, Vitreo Retinal Consultants, donated to the left-leaning Majority PAC last election cycle, reports the OpenSecrets.org website run by the nonpartisan Center for Responsive Politics.

Now, Melgen himself is feeling the heat.

Already hit by the Internal Revenue Service in 2011 with an $11 million tax lien, FBI agents arrived at the Melgen Retina Eye Center in West Palm Beach -- formerly called the Vitreo Retinal Consultants Eye Center -- after dark Tuesday to execute a search warrant.

The search, which lasted well into Wednesday, was sparked in part by the appearance of a shredding truck on the property, said a law enforcement source with knowledge of the investigation. Agents from the FBI and U.S. Health and Human Services Department left the offices carrying box after box of material.

An FBI spokesman in Florida has confirmed there was "law enforcement activity" at the location, but did not elaborate or mention any person specifically.

Melgen's lawyer denies wrongdoing by his client.

"The government has not informed Dr. Melgen what concerns it may have," Dean L. Willbur Jr. told CNN on Thursday, adding that the doctor's issues with the IRS "have been fully resolved."

"We are confident that Dr. Melgen has acted appropriately at all times," said Willbur.

Sen. Johnny Isakson, a Georgia Republican and member of his chamber's ethics committee, acknowledged Friday that his office is "aware of the news reports regarding the FBI raid on Dr. Melgen's office."

Menendez's spokeswoman, Patricia Enright, has said the New Jersey senator assumes the committee is reviewing the entire matter, but he has not been contacted.

"The Ethics Committee will follow its established procedures on this matter," Isakson said.

Menendez calls Melgen, who according to his official biography graduated from the Dominican Republic's National University and has been honored by its government, a friend and political supporter. Melgen and his family have frequently contributed to the New Jersey senator's causes -- contributing thousands to his campaigns, not to mention the Democratic Senatorial Campaign Committee he led from 2009 to 2011.

Menendez, set to become chairman of the Senate Foreign Relations Committee, is already facing questions over his travel on Melgen's plane.

He has said the 2010 Caribbean trips, which took place in 2010, were "paid for and reported appropriately," though his spokeswoman Enright admits it was an "oversight" for Menendez to take until January 4 of this year to pay $58,500 from his personal funds for two of the flights.

Another issue has to do with whether Menendez advocated on behalf of ICSSI -- a port security company that Melgen has a stake in -- on Capitol Hill.

During a Senate subcommittee hearing last July, Menendez didn't mention ICSSI by name, but he did press Deputy Assistant Secretary of State Matthew Rooney about an unnamed company who had a contract to X-ray cargo that went through all Dominican ports -- a contract that, he said, Dominican authorities "don't want to live by."

"If those countries can get away with that, they will," the senator said. "And that puts American companies at a tremendous disadvantage."

Reached Friday, Menendez's chief of staff Dan O'Brien -- while not mentioning any company specifically -- proudly noted his boss's advocacy "for more attention to the spread of narco-trafficking throughout Central America and the Caribbean."

"Stemming the growth of narco-trafficking is a key challenge in the region, and it is a fight from which Sen. Menendez will never back down," O'Brien said.

Not everyone supports ICSSI's contract in the Caribbean nation, as constituted. Foes include the American Chamber of Commerce of the Dominican Republic , a trade organization representing about 2,500 American and Dominican companies.

The group said that while it doesn't oppose "increased security measures ... we oppose the contract as conceived" -- claiming it gives ICSSI a "monopoly (in) violation of the Dominican constitution."

"They are not what we believe they should be, and they violate all sorts of international treaties," said the chamber, which further claims ICSSI's rates are exorbitant. "The ICSSI contract undermines national competitiveness, and there is no added value to it."