McCaskill's panel conducted the hearing jointly with Tester's subcommittee on efficiency and effectiveness of federal programs.
USIS, based in Falls Church, Va., said in a statement that it has never been informed that it is under criminal investigation. USIS received a subpoena from the inspector general's office in January 2012 for records, the statement said. "USIS complied with that subpoena and has cooperated fully with the government's civil investigative efforts," according to the company.
USIS declined to comment on whether it conducted a background investigation of Snowden. The company said it performs thousands of background investigations each year for OPM and other government agencies. "These investigations are confidential and USIS does not comment on them," the USIS statement said.
The background check USIS performed on Snowden was done in 2011 and was part of periodic reinvestigations that are required for employees who hold security clearances, according to McFarland and Michelle Schmitz, the assistant inspector general for investigations at OPM.
Schmitz said the investigation of USIS commenced later in 2011.
Booz Allen Hamilton, the company where Snowden was working at the time of the disclosures, fired him for violations of the firm's code of ethics and firm policy. The company said he had been a Booz Allen employee for less than three months.
Snowden worked previously at the CIA and probably obtained his security clearance there. But like others who leave the government to join private contractors, he was able to keep his clearance after he left and began working for outside firms.
Of the 4.9 million people with clearance to access "confidential and secret" government information, 1.1 million, or 21 percent, work for outside contractors, according to a January report from the Office of the Director of National Intelligence. Of the 1.4 million who have the higher "top secret" access, 483,000, or 34 percent, work for contractors.