The same company that investigated Edward Snowden for his security clearance in 2011 also did a background check on Navy Yard shooter Aaron Alexis in 2007. That’s no surprise — the company, USIS of Falls Church, dominates the background-check market. This is just the latest problem to dog the federal security clearance process. Brenda Farrell is director of defense capabilities and management issues at the Government Accountability Office. She has kept close watch on these issues.