Months after the FBI began probing allegations against Sen. Robert Menendez (D., N.J.), investigators are now looking at whether someone set out to smear him while he was running for reelection last year and then ascending to his new post as chairman of the Senate Foreign Relations Committee, according to four people briefed on the inquiry.

As part of a wider public-corruption investigation into the senator, the FBI has been examining whether Menendez patronized prostitutes in the Dominican Republic, according to people familiar with the inquiry. They said agents have been trying to determine whether the senator's longtime political supporter and friend, eye doctor Salomon Melgen, provided the women, free flights on his private plane, and any other services as illegal gifts.

But the inquiry into the prostitution allegations has come up dry, producing no evidence to back them up, say four people briefed on the probe. They, among others, spoke on the condition of anonymity to discuss an open investigation.

The broadening inquiry is drawing in a cast of characters whose travels have intersected with Melgen and Menendez in South Florida and the Dominican Republic.

The FBI has asked to interview a former CIA operative who is now a Florida businessman with interests in the Dominican Republic. Investigators have also recently sought to trace the cybertrail of an anonymous tipster who first made the prostitution claims in spring 2012 to a government watchdog group.

Meantime, Menendez continues to face a federal grand jury investigation into whether he improperly used his public office to help Melgen's business interests while taking gifts, people familiar with the probe said.

A spokesman for the FBI's Miami field office declined to comment.

Menendez and Melgen have repeatedly said they engaged in no wrongdoing. They have called the allegations involving prostitutes a baseless and nefarious attack designed to hurt the senator's reelection effort and undermine his clout.

"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," Menendez's spokeswoman Tricia Enright said.