(CNN) -- Investigators have been aware for some time of the allegations that Cuba's government tried to smear U.S. Sen. Robert Menendez, a U.S. law enforcement official said. They have found no evidence to support the claim that Cuba tried to smear him, said the official. Earlier, Menendez told CNN in an exclusive interview that the U.S. government should investigate any evidence it has that he was the subject of a conspiracy by the Cuban regime to spread false allegations that he traveled to have sex with underage Dominican prostitutes.