The two, who spoke on the condition of anonymity to discuss an active investigation, said FBI agents are following leads provided by an unknown tipster. In a series of e-mails with the FBI, the tipster alleged that the doctor, Salomon Melgen, had made prostitutes available to Menendez while he was staying at his friend’s resort home in the Dominican Republic. The tipster, in particular, mentioned young prostitutes and prostitution parties.

The FBI agents are also examining the role Menendez played in advocating for a port security contract in the Dominican Republic that would benefit Melgen, two people familiar with the case said. Menendez has urged U.S. officials to put pressure on the Dominican government to enforce the dormant contract, saying enhanced port security is important for interdicting drug trafficking. Melgen is an investor in the company holding the contract.