Terrorism has been ruled out as a motive in Wednesday’s fatal shooting at Fort Hood that killed three people before suspected gunman Spc. Ivan Lopez killed himself, but investigators are still unsure what caused Lopez to open fire on the Texas military base.

"We are digging deep into his background, any criminal history, psychiatric history, his experiences in combat, all the things you would expect are being done right now," said Lt. Gen. Mark Milley, commanding officer at Fort Hood, according to the BBC. Lopez was being evaluated for post-traumatic stress disorder and was undergoing treatment for depression and anxiety, the Associated Press reported.

Lopez, a veteran of the war in Iraq, was brandishing a .45-caliber semi-automatic pistol and fired into a medical support building around 4:30 p.m. Wednesday local time, according to the New York Times. He then got into a vehicle and opened fire from the vehicle. Lopez then got out of the vehicle and walked into another building before a military police officer took out her weapon. He then put his gun to his head and shot himself.

Milley praised the actions of the military police officer, saying she was “clearly heroic.”

“She did her job. She did exactly what we would expect of U.S. Army military police,” Milley told the Times.

Three people were killed and 16 others injured in the Fort Hood shooting. The incident evoked memories of 2009, when ex-Army psychiatrist Nidal Hasan committed a mass shooting at the same military base in an attack that killed 13 people and wounded 32. Hasan, who said he committed the attacks as an act of jihad, received the death penalty for the murders.

President Barack Obama said there will be an investigation into the latest Fort Hood shooting. He also referenced the 2009 attack while addressing the latest incident.

"They serve with valor. They serve with distinction, and when they're at their home base, they need to feel safe," Obama said of the soldiers who call Fort Hood home. "We don't yet know what happened [Wednesday], but obviously that sense of safety has been broken once again."