HOUSTON -

Investigators are trying to figure out why the man who opened fired at the Fort Hood military base killed three others and wounded 16 before turning the gun on himself.

Officials say the suspected shooter, identified as Spc. Ivan Lopez, was undiagnosed with post-traumatic stress disorder (PTSD) and undergoing some type of mental health treatment.

Lopez, along with his wife and children, moved to the Fort Hood base in February.

Officials say Lopez, 34, served in Iraq for four months in 2011.

He was never wounded in action, but officials say he had a self-reported traumatic brain injury upon returning to the United States.

Lopez had several mental health issues and was on medication, in addition to receiving psychiatric help for depression and anxiety, according to investigators. Lopez was also undergoing a process to determine if he had PTSD.

“It's a lot easier to look back than it is to diagnose ahead of time and know what the results of the diagnosis is going to be,” said David Maulsby, executive director at PTSD Foundation of America in Houston. “It's way too early to know exactly what he was dealing with and can they find out ahead of time. I don't think there's a way of knowing that right now.”

Maulsby said dealing with PTSD is quite difficult. Once PTSD is diagnosed, the treatment includes following up with the symptoms, helping the patient learn how to adjust, finding the triggers that set them off, and then helping them learn how to avoid those and cope with them a little better.

Maulsby said those diagnosed with PTSD are more likely to take their own lives instead of carrying out a mass shooting.

"PTSD causes someone to become more suicidal than homicidal," Maulsby said. "They feel useless, of no value, they isolate and that's when they start the downward spiral. And that's when they take their own lives, not others'."

Learn more by visiting the PTSD Foundation of America’s website.