WASHINGTON – The same federal contractor that vetted Edward Snowden, who leaked information about classified U.S. spying programs, also performed a background check that let the Washington Navy Yard shooter to obtain a security clearance.

Now the contractor, USIS, is drawing fire from a senator asking how Snowden and Navy Yard shooter Aaron Alexis slipped through the cracks. The vetting process also has been included in an inquiry by law enforcement agencies into Alexis' activities before his deadly rampage this week.

No company does more U.S. government background checks for clearances than USIS, which was awarded $253 million by the Office of Personnel Management last year. The company did about two-thirds of background investigations done by contractors, and more than half of all those performed by the U.S. personnel office, according to the office of Sen. Claire McCaskill, D-Mo.

"What's emerging is a pattern of failure on the part of this company, and a failure of this entire system, that risks nothing less than our national security and the lives of Americans," McCaskill said in a statement. "We clearly need a top-to-bottom overhaul of how we vet those who have access to our country's secrets and to our secure facilities."

Alexis, 34, obtained a secret-level clearance from the Navy in March 2008 that would have enabled him to get the access card he used to get on the base. After leaving the Navy in January 2011, Alexis kept the clearance even with three arrests, a history of mental illness and a record of military misconduct. His clearance was good for 10 years and wasn't subject to reinvestigation, according to a defense official who wasn't authorized to speak publicly.

Security specialists say USIS, a unit of Falls Church, Va.-based Altegrity Inc., owned by Providence Equity Partners, is a beneficiary of a ­system where the number of people with security clearances surged to nearly 5 million as of last year. Investigators are overworked and underpaid, security specialists say, and the government has become increasingly reliant on outside contractors to do background checks.

Merton Miller, associate director for federal investigative services at the Office of Personnel Management, said that the agency "has reviewed the 2007 background investigation file for Aaron Alexis, and the agency believes that the file was complete and in compliance with all investigative standards."