Citing what it said were "two people familiar with the investigation," the Post reported Friday agents are focusing on whether prostitutes were provided by Dr. Salomon Melgen, a Florida eye doctor and campaign contributor. One of the newspaper's sources said agents were looking into the possibility underage prostitutes were involved.
The report said the FBI got information from an anonymous tipster.
The investigation also involves Menendez's dealings with Melgen and whether the senator helped facilitate a contract for port security in the Dominican Republic that benefited the doctor, the Post said.
The Senate Ethics Committee has begun an investigation into whether Menendez accepted free plane trips and other gifts from Melgen.
Menendez reimbursed Melgen in December for two trips to the Dominican Republic on the doctor's private plane in 2010.