A team of FBI agents has been conducting interviews in recent weeks in the Dominican Republic and the United States, looking into allegations that Sen. Robert Menendez (D-N.J.) patronized prostitutes in the Caribbean nation, but has found no evidence to support the claim, according to two people familiar with the investigation.

One person said agents have asked about whether a Florida eye doctor — a close friend and major campaign donor to Menendez — provided the senator with prostitutes on vacations there. Another person said investigators are looking into allegations involving underage prostitutes and sex parties.

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.

FBI spokesman Michael Leverock, of the bureau’s field office in Miami, declined to comment for this article.

Menendez has vehemently denied the prostitution allegations, calling them false “smears.”

“As we have said all along, Senator Menendez welcomes any review because his actions have been appropriate, and we believe the facts will confirm that,” a Menendez aide said Friday.

The staff member, who spoke on the condition of anonymity, said the senator’s efforts on behalf of the port deal were appropriate. “The senator has had a long history of advocating for increased port security across the world,” the aide said.

Melgen and his attorneys declined to comment on any criminal investigation.

The relationship between Menendez and Melgen has come under intense scrutiny in recent weeks. Menendez acknowledged this month that he had failed to properly disclose two trips he took in 2010 on Melgen’s private plane to Melgen’s villa, near the resort of Casa de Campo. The senator said he wrote Melgen a personal check for $58,500 to reimburse him for the plane rides last month. The Senate Ethics Committee is investigating the matter.

Separately, Melgen is under criminal investigation over allegations of Medicare fraud. Three people with knowledge of the probe disclosed this week that it focuses on whether Melgen defrauded the government by overcharging Medicare for millions of dollars in eye injections and billing the government for treatments that were unnecessary.

The FBI and the Department of Health and Human Services’ Office of Inspector General raided Melgen’s Palm Beach eye clinic Jan. 29, searching records and collecting boxes of documents. The raid came days after the tipster’s e-mail exchange with the FBI was posted anonymously on a Web site.

Melgen’s attorney said this client was cooperating with authorities in the Medicare investigation and declined to answer further questions.

“At this time it would be inappropriate to comment about the ongoing investigation,” said the attorney, Kirk Ogrosky.

In checking into the prostitution allegations, FBI agents are examining claims initially made in a series of letters the tipster wrote last spring to staff members at a government watchdog group, Citizens for Responsibility and Ethics in Washington (CREW), according to the two people familiar with their inquiries.

The claims were also carried on a conservative news Web site, the Daily Caller.

CREW’s executive director Melanie Sloan said her staff could not get the tipster to agree to meet or speak on the telephone. As a result, Sloan told a Washington Post reporter last month, she had doubts about the credibility of the person’s claims. The FBI began looking into the tipster’s story after CREW forwarded his information to the Justice Department and the FBI’s Washington field office in July.

Prostitution is not illegal in the Dominican Republic. But soliciting underage prostitutes is illegal. Also, a politician who accepts free gifts and special favors in exchange for official acts can face criminal prosecution for abuse of public office.

The allegations in the letters included the names of a few women who allegedly had improper relations with Menendez and Melgen, including that of a Dominican teenager who told a local television station she had no involvement and had never left her village.

The Miami Herald reached another one of the women for an interview, and she explained that she worked for Melgen and considered him a generous person but did not elaborate. An attorney for the woman, Svitlana Buchyk, told a Post reporter Friday that the tipster’s claims about her are “absurd.”

“Ms. Buchyk is an actress and model attending to her career,” said her attorney, Gerald Greenberg. “Any allegation that she had an improper relationship with Senator Menendez or anyone else is sheer nonsense. The claims about her that have appeared in fringe-media sources are irresponsible and utterly absurd.”

Greenberg declined to discuss whether his client had been interviewed by law enforcement officials.

In an e-mail to the tipster in September, FBI agent Regino Chavez wrote, “If you can give me . . . information on all the young ladies who were victimized that would be helpful.” The agent repeatedly asked the tipster, who called himself Pete Williams and only corresponded by e-mail, to meet, but he declined.

According to FBI affidavits filed in court, Chavez was listed last year as a member of a task force based out of the Miami field office that specialized in tracking crimes involving abuse of minors, soliciting minors to engage in prostitution and sex trafficking.

Ernesto Londoño and Alice Crites contributed to this report.