Photo by Alex Wong/Getty Images.

The law enforcement officers who killed the Navy Yard shooter showed “exceptional valor,” an investigation has determined, and won’t face any charges.

The U.S. Attorney for the District of Columbia’s office determined through “witness statements, surveillance video, photographs, diagrams, physical evidence, law enforcement agency reports” and Aaron Alexis’ autopsy report that the force used against him was justified for self defense. “The review determined that the officers acted reasonably at all times to neutralize a life-threatening situation,” a release states.

Alexis, a 34-year-old Department of Defense contractor, killed 12 people at the Naval Sea Systems Command headquarters in September 2013 before being fatally shot by Metropolitan Police Department and Park Police officers. A release details the confrontation that led to this.

Mr. Alexis hid under a desk on the third floor, waited, and attempted to ambush an Emergency Response Team officer from the MPD and a U.S. Park Police officer as they entered the area where he was located. At about 9:25 a.m., Mr. Alexis shot at the MPD Emergency Response Team officer, hitting the plate of his police tactical vest. The MPD Emergency Response Team officer and the U.S. Park Police officer returned fire and were able to shoot and kill Mr. Alexis.

“After a careful review of the evidence, we have closed this investigation,” U.S. Attorney for D.C. Ron Machen said in a statement. “We concluded that the law enforcement officers involved demonstrated exceptional valor in acting to protect the lives of Navy Yard employees and other responding law enforcement officers.”

A report from the Department of Defense determined the mass shooting could have been prevented.