The Post reported Thursday among those questioned by agents have been Alfonso and Jose Fanjul, the Cuban-born sugar barons who are also the largest landholders in the Dominican Republic. The Fanjuls are major political donors, one to Democrats and the other to Republicans.
The stories surfaced as Menendez, D-N.J., was preparing to succeed Sen. John Kerry, D-Mass., as head of the Foreign Relations Committee.
The first allegations against Menendez, reported by the right-wing news site The Daily Caller, accused him of patronizing prostitutes in the Dominican Republic provided by friend and contributor, Salomon Melgen, a Florida opthamologist and businessman. But a tipster who corresponded with the FBI by email has refused face-to-face meetings and women have said they were coerced into telling false stories.
Menendez also faces accusations he used his influence to help Melgen with a port deal in the Dominican Republic.
Alphonso Fanjul actually called Menendez to deny that he and his brother were the source of the stories, the Post said. Roger Stone, a Republican operative based in New Jersey with a reputation for dirty tricks, has also denied originating the stories, although he has said he believes their truth is still "an open question."
Menendez has consistently denied any wrongdoing.
"Whoever organized and carried out the false smear campaign against Senator Menendez appears to have broken the law, and as we have said from the beginning, we believe this matter should be investigated fully," a spokeswoman, Tricia Enright, said.