Before he shot and killed 12 people in the Navy Yard rampage, Aaron Alexis had a string of arrests spanning years. That’s led many to wonder how he could’ve received and retained the security clearance that enabled him to enter the secure building in the first place. On Thursday, the company that performed the background check on Alexis in 2007 and the agency that oversaw the work issued separate statements in response to questions about the shooting. Here they are in their entirety: “The security clearance process begins when an agency identifies a person who will require eligibility for access…