(CNN) – The private firm that vetted Edward Snowden in 2011 is under criminal investigation for routine failures in properly investigating the backgrounds of people in line for security clearances, Sen. Claire McCaskill said during a Senate hearing Thursday.
Additionally, a government watchdog told lawmakers his agency believes the check into Snowden's background conducted by USIS, a Virginia-based government contractor, may have been faulty.
Snowden, who held a top secret clearance, admittedly leaked documents this month detailing two government surveillance programs. At the time of the leaks, he was an employee of defense contractor Booz Allen Hamilton.
McCaskill, a Missouri Democrat, didn't specify Thursday whether the criminal probe into USIS was prompted by Snowden's leaks. She was heading a hearing into the process of obtaining security clearances, which has come under scrutiny since Snowden's identity was made public nearly two weeks ago.
One of the witnesses before the lawmakers was Robert McFarland, the inspector general for the federal Office of Personnel Management, who in a statement to the committee detailed extensive fraud committed by investigators charged with vetting people before they're granted security clearances.
Asked Thursday by Sen. Jon Tester, D-Montana, whether USIS's look into Snowden's background "may not have been conducted in an appropriate or thorough manner," McFarland said there was reason for concern.
"Yes, we do believe there may be some problems," McFarland said.
In a statement, USIS said they could not comment on whether they conducted an investigation into Snowden, and added they have "never been informed [USIS] is under criminal investigation."
"USIS conducts thousands of background investigations annually for OPM and other government agencies. These investigations are confidential and USIS does not comment on them," the firm said.
Speaking before the committee, McFarland said the department that conducts background checks currently has a dangerously low level of supervision.
"The lack of independent verification of the organization that conducts these important background investigations is a clear threat to national security," McFarland said.
In his statement to the Senate panel, McFarland detailed so-called "fabrication cases," which occur when background investigators "report interviews that never occurred, record answers to questions that were never asked, and document records checks that were never conducted."
In one case, a woman responsible for conducting credit checks was found to have fabricated 1,600 different reports. In an ironic twist, the background check used to hire her was also found to be false.
In all, 18 employees have been criminally convicted of falsifying background checks, according to McFarland's statement.
Another investigator pleaded guilty to background check fraud last month, and yet another is expected to enter a similar plea this week.
A report in January from the Director of National Intelligence showed nearly five million people hold United States security clearances. They are broken into confidential, secret, and top secret classifications based on the sensitivity of the information a person is allowed to view.