Following Monday’s mass shooting at the Washington, D.C. Navy Yard and subsequent revelations that shooter Aaron Alexis had previous arrests, U.S. Senator Claire McCaskill is seeking answers from top federal officials on the thoroughness of his background investigation.

“I want to know who conducted his background investigation, if that investigation was done by contractors, and if it was subject to the same systemic problems we’ve seen with other background checks in the recent past,” said McCaskill, Chairman of the Subcommittee on Financial & Contracting Oversight. “While guilt ultimately lies with the perpetrator of this terrible crime, those who lost loved ones and were injured in this shooting deserve to know the answers to these questions.”

In a bipartisan letter to Office of Personnel Management Inspector General Patrick McFarland, McCaskill requested information on who performed Alexis’ background investigations, when those investigations took place, and if his arrest record was given proper scrutiny.

“(How) did Mr. Alexis’ background investigation address his pattern of misconduct, including his arrests on charges, relating to firearms in 2004 and 2010 and his arrest for disorderly conduct in 2009?” the letter reads.

Today’s letter follows a watchdog report released yesterday that detailed major deficiencies in a program that was used to grant contractor employees unrestricted access to Navy installations-to which McCaskill has demanded answers from the U.S. Navy Secretary. McCaskill previously held a Senate hearing on oversight failings occurring in security clearance background checks, in which it was revealed that the company USIS, which had conducted Edward Snowden’s investigation was under criminal investigation.

Military service members, federal workers, and contract personnel must obtain a security clearance to gain access to classified material. In 2012, there were 3.5 million federal employees and 1.1 million contractors who held a Secret or Top Secret Clearance. The Office of Personnel Management (OPM) Federal Investigative Services Division handles almost all background investigations for security clearances for federal agencies. OPM’s security clearance and background investigations cost the federal government approximately $1 billion in 2012-a yearly cost that is expected to rise to $1.2 billion by 2014.

OPM spends 46 percent of its funds on the contractors who perform investigations. Approximately 75 percent of all field investigators are contractors-an estimated 4,600 out of 6,200 in total.

McCaskill’s letter was joined by Subcommittee on Financial and Contracting Oversight Ranking Member Ron Johnson (R-Wis.), and Subcommittee on the Efficiency and Effectiveness of Federal Programs and the Federal Workforce Chairman Jon Tester (D-Mont.) and Ranking Member Rob Portman (R-Ohio).