Democratic Sen. Bob Menendez under federal investigation: Sources

Sen. Bob Menendez, D-N.J., is under federal criminal investigation, sources familiar with the matter told ABC News and confirmed by an adviser to the senator.

The investigation is being conducted by federal prosecutors in Manhattan, the sources said. The exact nature of the investigation was not immediately clear.

"Senator Menendez is aware of an investigation that was reported on today, however he does not know the scope of the investigation. As always, should any official inquiries be made, the Senator is available to provide any assistance that is requested of him or his office," said the adviser, Michael Soliman, in a statement provided to ABC News.

A spokesman for the U.S. Attorney's Office for the Southern District of New York declined to comment, as did representatives of the FBI.

The news was first reported by Semafor.

Menendez, who is chairman of the powerful Senate Foreign Relations Committee, was previously indicted on corruption charges in April 2015 due to his relationship with a Florida eye doctor, Salomon Melgen.

Prosecutors accused Menendez of accepting close to $1 million worth of campaign contributions and lavish gifts -- flights on Melgen's private jet, a first class commercial flight and a flight on a chartered jet; numerous vacations at Melgen's villa in the Dominican Republic and a hotel room in Paris -- from Melgen in return for the political favors. Menendez also allegedly used his office to support the visa applications of several of Melgen's girlfriends.

He pleaded not guilty. The trial ended with a hung jury and the charges were dismissed in 2018.

Melgen was later convicted of defrauding Medicare patients but had his prison sentence commuted by former President Donald Trump in his final hours in office.