Sen. Robert Menendez' office says he has previously reimbursed to a West Palm Beach doctor the cost of two trips they made to the Dominican Republic on the doctor's private plane.
Menendez' office says the senator paid Dr. Soloman Melgen $58,000 from his private account, which is why the reimbursement was not previously reported.
FBI agents are beginning to piece through documents they removed Wednesday morning from a Melgen's West Palm Beach office.
Published reports say the FBI is investigating whether Melgen helped hook the senator up with underage prostitutes.
"In a case like this, they're probably looking for receipts, they're probably looking for travel itineraries, they're probably looking for logbooks, diary entries, anything that can show where certain people were on certain days," said Jonathan Wasserman, a former prosecutor and criminal defense attorney.
Documents obtained by NewsChannel 5 show the West Palm Beach ophthalmologist gave at least three plane rides to Senator Menendez to the Dominican Republic on his private plane.
"Cases like these are hard to build. They need a lot of information, they need witnesses to corroborate the information they have. It's going to take more than, 'we think this is what happened, '" said Wasserman.
Earlier today, Sen. Menendez released a statement saying "Dr. Melgen has been a friend and political supporter of Sen. Menendez for many years. Sen. Menendez has traveled on Dr. Melgen's plane on three occasions, all of which have been paid for and reported appropriately. Any allegations of engaging with prostitutes are manufactured by a politically-motivated right wing blog and are false."
Melgen's lawyer said nothing at the doctor's office Wednesday.
And Wasserman, who is now a defense attorney, suspects the FBI is remaining silent because building a case takes time.
"What they're probably hoping to find is evidence to further their investigation that could lead to other names, other locations, it could lead to the government finding other people to see what they can find to build a case, " said Wasserman.
The Associated Press contributed to this report.