Dr. Salomon Melgen, an eye surgeon, flew Menendez around on his private jet and contributed hundreds of thousands of dollars to benefit him and the national Democratic Party, The New York Times reported Friday.
Menendez also provided support to Melgen, records and interviews indicated, urging Cabinet officials to intervene in a contract dispute between a security company Melgen owned and the Dominican Republic. The contract was valued at $500 million.
An aide told the Times Menendez did nothing improper in advocating for Melgen's business interests.
FBI agents raided Melgen's West Palm Beach, Fla., offices, removing dozens of boxes of documents and other materials.
Menendez, whose friendship with Melgen goes back to the 1990s, finds he must defend his relationship with a key political supporter as he nears the most consequential post of his political career -- chairmanship of the Senate Foreign Relations Committee, which opened when John Kerry was confirmed as secretary of state, the Times said.
Melgen was a major contributor to the political campaigns of Menendez and others, the report said.
The Center for Responsive Politics said Melgen contributed more than $200,000 to mostly Democratic candidates and political committees since 1990.
An FBI spokesman declined to tell the Times the focus of the inquiry.
"The government has not informed Dr. Melgen what its concerns are," the doctor's lawyer said.
"However, we are confident that Dr. Melgen has acted appropriately at all times."
Menendez declined to be interviewed, the Times said.
His office issued a statement that read in part, "Dr. Melgen has been a friend and political supporter of Senator Menendez for many years."