Essential personnel were allowed to return to the Washington Navy Yard on Tuesday, and most streets in the area were reopened to traffic, as investigators searched for clues as to why a former Navy reservist entered the base with a shotgun on Monday and killed a dozen people before dying in a gun battle with police.

The FBI identified the shooter as Aaron Alexis, 34, of Fort Worth, who had been hired by a private contractor to do a project at the Navy Yard, and had a security clearance and a valid military identification card, known as a common access card, which allows unfettered access into most facilities.