The agency did not conduct a new background check either last year, when Alexis was first hired as a military contractor, or this summer, when Alexis was seeking a second stint with the contracting firm, people familiar with his hiring said. Instead, the company hired an approved background screening firm that confirmed with the Defense Department that Alexis’s security clearance remained valid.

A secret-level clearance requires far less-intensive digging than one for top-secret clearance, and involves only a check of the FBI database, military records and data from law enforcement agencies where an applicant has lived, worked or attended school during the past five years. It does not require interviews with family members, co-workers or employers.