The number of people thought to be involved in the alleged “unmasking” of American citizens under the Obama administration could be expanding, according to a source close to the House Intelligence Committee’s review.

The source with knowledge of the review told Fox News the records suggest the unmasking “goes beyond” key officials like former National Security Adviser Susan Rice, former CIA Director John Brennan and former U.S. Ambassador to the U.N. Samantha Power.

The source said more than a half-dozen former senior Obama administration officials are now of interest to House committee investigators.

Committee Chairman Devin Nunes, R-Calif., issued subpoenas in May to the CIA, FBI and NSA for records about the identification of American citizens in intelligence reports, also known as “unmasking.”

The subpoenas covered 2016 and were issued after allegations Rice and others unmasked associates of now-President Trump. Fox News is told the agencies have now “fully complied.”

While Nunes has taken himself out of the committee’s Russia inquiry following an ethics complaint which he has denied, the chairman remains involved in the unmasking inquiry. Committee Democrats have criticized Nunes for issuing subpoenas in his current capacity.

Separately, the committee has been preparing to hear from Rice in a closed-door session as early as Tuesday, though the former national security adviser is no longer expected to testify that day.

The “unmasking” issue is likely to be a central focus of her eventual testimony.

The source said some of the unmasking in question could be routine – though other requests appeared “unwarranted” and will require “more explanation.” The source said House investigators are only at the beginning of their assessment and described the paper trail as containing “a significant number of records.”

The congressional scrutiny at this stage does not mean investigators have concluded any official violated the law or internal regulations.

Rice spokeswoman Erin Pelton previously told the Associated Press that Rice is cooperating with House and Senate intelligence committees.

Rice in March had told PBS she “knew nothing” about Trump associate unmasking.

But in early April, she acknowledged she asked for the identities of U.S. citizens in intelligence reports, while defending those requests as routine and denying leaking any Trump-related information.

"I leaked nothing to nobody, and never have," she told MSNBC.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.