Federal investigators could file criminal charges against U.S. Sen. Robert Menendez, D-N.J., this week, the Wall Street Journal reported Sunday, citing anonymous sources. Though details of the charges weren't specified, they’re believed to stem from an ongoing investigation into whether the ranking Democrat on the powerful Senate Foreign Relations Committee inappropriately took gifts from a Florida eye doctor facing an investigation into his billing practices.

Earlier this month Menendez denied any wrongdoing, saying he’s “not going anywhere,” after news emerged that the U.S. Department of Justice was preparing to file criminal corruption charges against the senator, accusing him of promoting the interests of the eye doctor -- a party donor and personal friend -- in exchange for gifts.

The Justice Department has been looking into Menendez’s activities for more than two years. After the investigation began, the senator reimbursed the eye doctor, Salomon Melgen, $60,000 for the cost of two round-trip flights on a private jet to the Dominican Republic. Menendez didn’t initially report the trips on disclosure forms, as required by law.

The FBI is looking into whether Melgen sought help from Menendez in connection with the billing investigation. The doctor is being investigated for allegedly overbilling Medicare for eye procedures. Melgen has donated hundreds of thousands of dollars to the campaigns of Menendez and other Senate Democrats, the Journal reported, citing election records.

Melgen’s lawyer has previously said that his client is innocent of any inappropriate behavior in his relationship to the senator. The Justice Department claims Menendez and his staff advocated on behalf of Melgen in meetings with senior Obama administration health officials. Menendez is also accused of trying to convince Department of Homeland Security officials in 2013 not to provide cargo-screening equipment to fight drug trafficking through the Dominican Republic. Melgen controlled a company that was vying to supply the equipment.

Two of Menendez’s aides have fought to avoid testifying to a grand jury under a “speech and debate” clause of the Constitution that protects members of Congress and their staffers from being questioned about legislative acts.

Menendez, who was re-elected in 2012, has been a vocal critic of President Barack Obama’s efforts to improve U.S.-Cuba relations and to increase dialogue with Iran over its nuclear program.