Fort Hood Shooting: Why Did a Soldier Open Fire?

Authorities are finding out why a married solider with a family went on a shooting rampage.
3:00 | 04/04/14

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Transcript for Fort Hood Shooting: Why Did a Soldier Open Fire?
Smiling pictures of a young father of three portray nothing of the mayhem to come. What made Ivan Lopez snap and fire on fellow servicemen and women at ft. Hood? New details about the shooter and the female officer who stopped him before he could kill again. We're on the ground, leading the coverage tonight. Reporter: Tonight, the burning question in this close-knit military community shattered again by a mass shoot, who was the man behind these pictures. Ivan Lopez, here he is smiling on photos posted on Facebook, his children by his side. A father of three, a husband, a soldier. Did something inside snap? Lopez was a 34-year-old army specialist with a clean record. He served four months in Iraq in 2011. The Pentagon says he never saw combat. But authorities say what he created here was a horror of its own. Shortly after 4:00 P.M. Yesterday, he opens fire on the base, 4:16, the first wounded soldiers call 11. We have an active shooter in ft. Hood. He gets in his vehicle and keeps fire pg. We have multiple gunshot victims. He then walks into a second building opening fire again. The alert goes out. Please shelter immediately. A tweet from ft. Hood, all personnel and post are asked to shelter in place. There may have been a verbal altercation with another soldier or soldiers and there's a strong possibility that that in fact immediately preceded the shooting. Then the critical moment in this parking lot when he's con fronted by that female military police officer. He puts his hands up at first and when he reaches inside his jack, she pulls out her guns and fires. He shoots himself in the head. He is down, he is down. There were several instance here of clear heroism. I would cite that female mp, with we're not going to release her name yet. She's germane to the investigation. But she clearly performed her duty exceptionally well. Tonight, we learned there were many hero, the chaplain inside shielding the soldiers. But when it was done, he killed three and wounded 16 other, three of them critically. They were brought in right through here? Right. Dr. Davis answered the call five years ago after the shooting here at ft. Hood. You never thought you would see this twice. When I heard the phone call that it was happening, it was shock and disbelief. He brought us into the er where he's sadly treating the wounded again. What was the worst of what you saw? Several I would consider pretty severe injuries injuries to the spine, the neck. We had chest wounds, abdominal wounds, et cetera? Too gunshots? Correct. Reporter: Among them, Keisha fountain. Her family said she tried to talk to the shooter. She was shot to the stomach. They were by her side as she underwent surgery today. This evening, the hospital telling us she made it through. She's been upgraded from critical condition to stable. And there's Carol miller's son, too. Patrick miller among the injured. These families all want to know what could have led to this. On the same Facebook page a post in 2013 showing Lopez in full gear and a comment. Memories of fallujah, Iraq, 2011. Authorities say he later diagnosed himself with draw mattic brain injury. He was not a wounded warrior. He was not wounded in action in our records. Lopez moved to the base in el Paso in ft. Hood with his second wife and a 3-year-old daughter. Neighbors say they watched him move in. No red flags, no warning signs. You saw them. They looked happy? Is in fact, neighbors saw his wife yesterday when news broke of the shooting. At first, she was worried about her husband's safety. She was crying. I consulted her. We all came down stairs. And as soon as they announced the name of the shooter, she just lost it. And what did she say to you? When a did she say to her? She said that's him, that's him. And I didn't really say anything to her, because what could I say. Here's what we did know. He purchased a .45 caliber handgun March 1, a little more than a month ago, the same gun store where major Nadal hassan bought his gun before killing 13 and injuring more than 30 here. And while authorities were aware Lopez suffered from mental issue, today in Washington the secretary of the army said there were no warning signs of anything like this. As of this morning, we had no indication on the record of that examination that there was any sign of likely violence, either to himself or to others. Today, my colleague spoke to Lopez's former supervisor in put reierto Rico who was full of nothing but praise. You would say he was an outstanding soldier? That's correct. He was an outstanding marine soldier. Reporter: But it was last summer when things began to unravel. He was undergoing a variety of treatment and diagnoses for mental health conditions ranging from depression to anxiety to some sleep disturbance. He was prescribed a number of drugs to address those, including am bee yen. He was being tracked for signs of post traumatic stress. It was not I see you for the first time and post traumatic stress disorder. Normally it's not a diagnosis you make on one appointment. It's something you make over a series of appointments. Reporter: New clues as authorities continue their investigation. We have strong evidence that he had a medical history that indicates unstable psychiatric or psychological conditions. Going through all the records to ensure that that is, in fact, correct. And we believe that to be the fundamental underlying cause factor. A family spokesperson said he was upset he was only granted a short leave to attend his mother's funeral last year and no one knew he was being treated for mental illness. How was he able to carry his gun on base and inflict so much destruction in 15 minutes of chaos. Ft. Hood, Texas. Up next on "Nightline,"

This transcript has been automatically generated and may not be 100% accurate.

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