March 18

ACCOMPANYING THE horror and grief that follow a mass shooting is the inevitable question of whether anything could have prevented the violence. In the case of last year’s shootings at the Washington Navy Yard, the answer is yes. That is the heartbreaking conclusion of a Defense Department review released Tuesday. Hopefully lessons learned from this will speed changes to help prevent future tragedies.

The reportexamined the events that resulted in the slaying of 12 Navy civilian and contractor personnel on Sept. 16. It found “missed opportunities for intervention” that could have prevented the gunman, Aaron Alexis, a former Navy reservist working for a private contractor, from retaining the security clearance that gave him unrestricted access to the Navy Yard’s Building 197.

Warning signs about Alexis’s mental health were overlooked or ignored. Federal officials signed off on his security clearance despite questionable gaps, and the information technology firm that employed him failed to report disturbing behavior Alexis exhibited in the weeks leading up to the shooting.

Among the report’s recommendations are sensible suggestions to improve the security-clearance process by insisting on more information, routinely updating background checks and setting up a system to evaluate and handle employees who may be a threat. Removing the stigma often associated with seeking mental health treatment is a continuing challenge. The report also suggests that too many workers have security clearances who don’t need them, thus diluting resources.

What’s significant — and disturbing — about the Navy Yard shooting is that it’s not the first time the military has faced a danger to its personnel and facilities from within. The Pentagon report and accompanying internal review noted with alarm other cases of threats posed by trusted insiders, including the 2009 Fort Hood shooting in which Maj. Nidal M. Hasan shot and killed 13 people and wounded dozens more and the incident in 2012 in which Spec. Ricky Elder fatally shot his battalion commander at Fort Bragg and then killed himself. It’s time, the report says, for the military to rethink outdated security theories that focus only on external threats and strong perimeters and to recognize the potential for terrorism and mayhem from within.