A friend of and political donor to Sen. Robert MenendezRobert MenendezCarson likely to roll back housing equality rule Live coverage: Tillerson's hearing for State Booker to join Foreign Relations Committee MORE (D-N.J.) is under investigation by the FBI, according to a report Wednesday in the Miami Herald.

According to the paper, FBI agents gathered at the medical office in West Palm Beach of Dr. Salomon Melgen, a contributor to Menendez and the owner of Casa de Campo, a luxury resort in the Dominican Republic. Melgen has on at least one occasion provided a private plane to allow Menendez to travel, and a spokeswoman has described the pair as longtime friends.

Unclear is whether the current FBI investigation is targeting Melgen for outstanding back taxes or unverified reports of providing improper benefits to Menendez.

According to the Herald, Melgen has an outstanding IRS lien of $11.1 million in back taxes owed from a three-year period. Melgen had previously been subjected to a $6.2 million lien that was released in 2011.

But Melgen's name also surfaced in a report by the conservative website The Daily Caller, which published an interview in November with Dominican prostitutes who claimed to have met Menendez at his resort. The prostitutes said Menendez paid them for sex, but the New Jersey lawmaker's office has vehemently denied the accusations.

“Dr. Melgen has been a friend and political supporter of Senator Menendez for many years," Menendez's office said in a statement. "Senator 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.”

The Melgen family is a frequent financial contributor in Florida politics, having given money to prominent members of Congress, including Sen. Bill NelsonBill NelsonLive coverage: Senators grill Trump's Treasury pick Trump's Commerce pick admits to unknowingly hiring undocumented worker Senate Democrats brace for Trump era MORE (D) and Reps. Debbie Wasserman Schultz (D), Joe GarciaJoe GarciaFreshman Curbelo wins reelection in Fla. LGBT Republican groups campaigning for Curbelo in Fla. House Democrats amplify anti-Trump strategy MORE (D), Ileana Ros-Lehtinen (R) and Mario Diaz-Balart (R).

-This post was updated at 2:28 p.m.