A private company that conducted a background check in 2011 on Edward Snowden, the former defense contractor who leaked sensitive information about the National Security Agency’s surveillance programs, has been under federal investigation for the past several months, a government watchdog testified at a Senate hearing on Thursday.

Patrick McFarland, inspector general of the U.S. Office of Personnel Management, or OPM, said at a Senate hearing that the security clearance check for Snowden, carried out by a Northern Virginia-based private firm, USIS, may have been faulty, Associated Press reported.

McFarland declined to reveal the reason or nature of the probe against USIS, which was established in 1996 as a result of the privatization of the investigative branch of OPM, a federal agency. 

USIS undertakes contracts from OPM to conduct background checks for more than 95 federal agencies and currently has 100 federal contracts, according to the company website.

Snowden was vetted by USIS in 2011 as part of periodic reinvestigations on federal employees with security clearances, and the investigation against USIS began later in 2011 in a “complicated contract fraud case," the Washington Post reported, citing OPM officials.

Sen. Claire McCaskill (D-Mo.), chairwoman of the Senate subcommittee on financial and contracting oversight, said she was told the USIS is facing a criminal probe for “systemic failure to adequately conduct investigations under its contract,” AP reported.

“We are limited in what we can say about this investigation because it is an ongoing criminal matter,” McCaskill said. “But it is a reminder that background investigations can have real consequences for our national security.”

In a statement on Thursday, USIS said it "has never been informed that it is under criminal investigation. In January 2012, USIS received a subpoena for records from the OPM’s Office of Inspector General (OIG). USIS complied with that subpoena and has cooperated fully with the government’s civil investigative efforts." 

The statement added that the company's investigations are confidential, and that it cannot say whether it conducted the original check or any subsequent background checks on Snowden.

Reuters reported on Thursday, citing anonymous sources, that officials in charge of recruitment at Booz Allen Hamilton, the NSA contractor that employed Snowden after his stint at the CIA, came across possible discrepancies in Snowden’s resume but decided to overlook them.

“Some of the educational information on the resume did not check out precisely,” sources told Reuters, but it remains unclear how Snowden convinced the firm to hire him.

Booz Allen Hamilton hired Snowden at an annual salary of $122,000 and placed him in Hawaii, where he worked for about four weeks before traveling to Hong Kong in May and leaking U.S. national secrets to a British newspaper.

Booz Allen Hamilton said in a statement that “we will work closely with our clients and authorities in their investigation of this matter,” Reuters reported.