(CNN) -- The FBI has begun a preliminary inquiry into allegations by the CIA that Senate staffers obtained unauthorized access to classified documents related to the agency's now-defunct post-9/11 interrogation program, according to people familiar with the matter.
The CIA's charges, made in criminal referrals to the Justice Department, are in contrast to Senate Intelligence Committee Chairman Dianne Feinstein's accusations last week that the agency spied on computers used by committee staffers at a CIA facility to investigate the Bush-era interrogation program.
The committee staff has produced a 6,300-page report on the program, which critics say violated prohibitions on torture and exceeded legal guidance from the Justice Department.
But the report remains in limbo, awaiting a declassification review by the CIA, which has disputed some of Senate findings.
The unusual dispute has raised tensions between the agency and lawmakers charged with overseeing the agency's activities.
Feinstein is usually one of the intelligence community's staunchest allies in Congress.
Justice Department officials have tried to avoid getting drawn into the fight, and some lawmakers have questioned whether it is appropriate for Justice, as an agency in the executive branch, to probe a dispute between legislators and another agency.
The FBI's preliminary inquiry is intended to determine whether a full-fledged investigation is warranted, people familiar with the matter said.
McClatchy Newspapers first reported the FBI inquiry.
CIA Director John Brennan has disputed Feinstein's accusations.
Speaking at the Council on Foreign Relations last week, he said "when the facts come out on this, I think a lot of people who are claiming that there has been this tremendous sort of spying and monitoring and hacking will be proved wrong."
Documents that prompted the dispute relate to an internal review by former CIA Director Leon Panetta and, according to the agency, were intended to help summarize material it was providing to the committee.
The documents were plainly labeled as for internal use and were not supposed to be reviewed by the committee.
Feinstein said Senate staffers found the documents in the course of their work that were put in the computer system either on purpose by a whistleblower or perhaps in error, and that they corroborated some of the committee's findings that the agency now says it disagrees with.
She said committee staff routinely sees such documents and didn't violate any classified restrictions.
Dean Boyd, a CIA spokesman, said in an op-ed published in USA Today that the agency acted properly after it discovered Senate staffers may have accessed and retained sensitive documents stored in a CIA computer network.
"These documents were privileged, deliberative, pre-decisional executive branch material that implicated separation of powers concerns," Boyd wrote. "Because we were concerned that there may have been a breach or vulnerability in the CIA local area network on which CIA stored these documents, CIA information technology specialists were asked to conduct a limited review to determine whether these files were located on the side of the CIA network the committee was authorized to use. That review appeared to confirm the committee's unauthorized access to the documents."