A group of bipartisan senators asked the government's Office of Personnel Management to investigate the steps that were taken to grant Aaron Alexis the security clearance that allowed him to gain entrance to the Navy Yard where he killed 12 people on Monday.

In a letter sent to the OPM, Sens. Ron Johnson, R-Wis., Claire McCaskill, D-Mo., Rob Portman, R-Ohio, and Jon Tester, D-Mont., submitted the request and highlighted reports that Alexis had previously been arrested for firearms violations.

"We are writing to request that your office review the security clearance background investigation(s) conducted for Aaron Alexis, the alleged perpetrator of the mass killing of at least twelve people at the Navy Yard complex in Washington, D.C., on September 16, 2013," the senators wrote. "Mr. Alexis reportedly was a Navy subcontractor employee with a history of misconduct, including previous arrests relating to the use of firearms."

The senators are specifically interested in whether the background investigations for security clearance "addressed his pattern of misconduct, including his reported arrests on charges relating to firearms in 2004 and 2010, and his arrest for disorderly conduct in 2009."

The senators are also asking the agency to determine Alexis' clearance level and status at the time of the shooting, who performed the background investigation, which agencies granted Alexis his initial clearance and subsequent clearance renewals, and whether the proper procedures were followed for investigating his background.

On Tuesday, several senators called for Senate hearings to investigate the security clearance process for federal contractors.