The FBI declined Wednesday to provide House Oversight Chairman James Comer with an internal law enforcement document that some Republicans claim will show Joe Biden was involved in an illegal scheme involving a foreign national.

"You have asked for what you say is a 'precise description' of an 'alleged criminal scheme' contained in is a single FD-1023 report. You express concern that the FBI has inappropriately 'failed to disclose' such a report 'to the American people,'" Christopher Dunham, acting assistant director for the FBI's office of congressional affairs, said in the letter to Comer, a copy of which was obtained by CNN.

"It is critical to the integrity of the entire criminal justice process and to the fulfillment of our law enforcement duties that FBI avoid revealing information — including unverified or incomplete information — that could harm investigations, prejudice prosecutions or judicial proceedings, unfairly violate privacy or reputational interests, or create misimpressions in the public," he continued in the letter.

FBI Director Christopher Wray declined during his testimony before the Senate Appropriations Committee on Wednesday to confirm whether the document exists.

"I will tell you that we understand completely the importance of congressional oversight, that's important to me," Wray said. "I also understand very much, as I think you do, the importance of us protecting sources and methods and ongoing investigations."

The White House has previously slammed the unverified claim against Biden, calling it another one of Republicans' "unfounded politically-motivated attacks."

In a six-page response to Comer, Dunham said that FD-1023 form is "used by FBI agents to record unverified reporting from a confidential human source," and noted that there are strict Justice Department guidelines about when that information can be provided outside of the FBI.

Dunham also outlined the shortcomings of relying solely on this law enforcement document -- which includes unverified allegations -- to draw conclusions.

"An FD-1023 form documents information as told to a line FBI agent. Recording the information does not validate the information, establish its credibility, or weigh it against other information known or developed by the FBI," Dunham wrote. "The mere existence of such a document would establish little beyond the fact that a confidential human source provided information and the FBI recorded it. Indeed, the FBI regularly receives information from sources with significant potential biases, motivations, and knowledge, including drug traffickers, members of organized crime, or even terrorists."

Dunham added that such documents include "leads and suspicions" and "later and more complete reports might exonerate individuals cast in a negative light by reports from earlier in an investigation."

Comer and Sen. Chuck Grassley, an Iowa Republican, claimed they had spoken to an unnamed whistleblower who claimed the FBI document included evidence that Biden, while vice president, was allegedly involved in an illegal payment that also involved a foreign national. Comer subpoenaed the document, setting a Wednesday deadline for the FBI to provide it. Pressed in media interviews for more details about the whistleblower's claims, Grassley has admitted he does not know whether they are true or false.

"It's clear from the FBI's response that the unclassified record the Oversight Committee subpoenaed exists, but they are refusing to provide it to the Committee," Comer said in a statement Wednesday. "We've asked the FBI to not only provide this record, but to also inform us what it did to investigate these allegations. The FBI has failed to do both. The FBI's position is 'trust, but you aren't allowed to verify.' That is unacceptable. We plan to follow up with the FBI and expect compliance with the subpoena."

Grassley added in a statement that "today's letter from the FBI raises additional questions, including whether the FBI has an open investigation based on these allegations."

Dunham said in his letter to the committee that the FBI was willing to coordinate with committee staff to "to discuss whether and how we can accommodate your request without violating our law enforcement and national security obligations."