The FBI will not confirm or deny the existence of records showing whether the bureau paid the researcher behind the unverified, controversial dossier alleging ties between President Donald Trump and Russia.

Christopher Steele, a former British spy, gathered information for the dossier while working for a Washington research firm that supporters of Hillary Clinton's presidential campaign hired. Trump has denied that the dossier is true while Steele has said parts of it are unverified.

The Washington Post reported earlier this year that Steele had reached an agreement with the FBI a few weeks before Election Day in November to continue investigating then-candidate Trump.

Cause of Action Institute, or CoA, a nonprofit watchdog group, wanted to know whether Steele was ever paid by the FBI to probe Trump.

In March of this year, CoA filed a Freedom of Information Act request to the FBI seeking access to records to determine whether the FBI paid, or intended to pay, Steele. CoA filed a lawsuit in federal court the following month against the FBI for not responding to the request.

The FBI replied to CoA last week in a letter, writing that the agency cannot speak to the potential existence of such records.

"The FBI can neither confirm nor deny the existence of records responsive to your request," the letter, signed by an official in the bureau's Records Management Division, said.

The FBI argued that it cannot acknowledge the existence of or give access to potential records concerning financial transactions with Steele because the FOIA request "implicates records the FBI may or may not compile pursuant to its national security and foreign intelligence functions." The bureau provided other reasons for its decision not to disclose information, including concerns over privacy and interfering in ongoing federal investigations.

Cause of Action Institute president and CEO John Vecchione released a statement castigating the FBI's response.

"The FBI is circling the wagons by claiming potential harm to national security if it discloses its relationship with Christopher Steele," Vecchione said. "Regardless of whether a payment was ever made, the FBI's affiliation with a political opposition researcher in the midst of a presidential election deserves scrutiny."

"The FBI should be forthcoming about whether and how the agency was relying upon a former foreign spy who, in the pay of private parties, compiled a report of salacious accusations intended to harm the reputation of then-candidate Donald Trump," Vecchione added.