The scope of the investigation into Menendez is unknown, but at least one subpoena has been sent in the case, the report said, citing people familiar with the matter.
The senator from New Jersey and his office staff are still available to provide official assistance as requested, Menendez's adviser Michael Soliman was quoted as saying.
Menendez serves as chairman of the Senate Foreign Relations Committee and is not up for re-election until 2024.
The senator was indicted in 2015 on federal corruption charges for accepting favors from an eye doctor, including hundreds of thousands of dollars in political donations to himself and the Democratic Party in exchange for government contracts, the report said.
Jurors were unable to reach a unanimous verdict in the case, leading to a mistrial in 2017, the report said.
The new case is similar in its broad outline to the previous one but the investigation this time around involves different parties, the report added.
In 2018, the US Senate Ethics Committee admonished Menendez after determining his actions violated chamber rules, federal law and standards of conduct.