As we come on the air, we're learning new details about the moment of crisis at ft. Hood military base in Texas, where the war veteran picked up a gun and started shooting. And we also know more,... See More
As we come on the air, we're learning new details about the moment of crisis at ft. Hood military base in Texas, where the war veteran picked up a gun and started shooting. And we also know more, tonight, about his mental state. And we know more about the heroes, too, who sprang into action to stop him. Ordinary Americans with great courage. A policeman and a cleric. So, we begin tonight, with ABC's David Muir, on the ground, at ft. Hood, taking us through what happened, moment-by-moment. David? Reporter: Diane, good evening. We've been on the scene all day. And this is a community stunned tonight. This evening here, we're learning more about who the shooter was. And who the hero was who stopped him. And the horror. Tonight, so many here asking who was the man behind these pictures? Ivan Lopez, dressed in his uniform. And here he is filing, on photos posted on Facebook. His children by his side. A father of three. A husband, a soldier. But something inside, snapped. 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 911. Then, he gets into his vehicle and keeps firing. He then walks into a second building, opening fire again. The alert goes out. Seek shelter immediately. Reporter: A tweet from ft. Hood. All personnel on post are asked to shelter in place. Then, the critical moment in this parking lot, when he's confronted by that female military police officer. He puts his hands up at first. And when he reaches inside his jacket, she pulls out her gun and fires. He shoots himself in the head. It was clearly heroic what she did at that moment in time. And she did her job. And she did exactly what we would expect of a United States army military police. Reporter: Tonight, we are learning there were many heroes. The chaplain inside, shielding the soldiers. But when it was done, he did kill 3 and wounded 16 others, 3 of them critically. They were brought in through here. Dr. Matt Davis answered the call after the shootings here in ft. Hood. He brought us into the E.R., where he is, sadly, treating the wounded again. What is the worse you saw? There were several that were severe injuries. Injuries to the spine. The neck. Reporter: Among them, 29-year-old Keisha fountain. She was shot in the stomach. They're by her side, undergoing surgery again today. And Carole miller's son, too. Major Patrick miller, among the injured. These families 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 later say he diagnosed himself with traumatic train injury. He was not wounded in action, to our records. No purple heart. Not wounded in action, in that regard. Reporter: Lopez had moved from a base in El Paso here to ft. Hood, just three months ago, with his second wife and their 3-year-old daughter. Neighbors, today, telling me, they watched the family move in. You saw them. And they looked happy? Yeah. Seemed happy. Reporter: Neighbors were with his wife yesterday, when news crossed of the shootings. At first, she was worried for her husband's safety. She was crying. So, I just, you know, I just consoled her. You know, I thought everything would be okay. And we all came downstairs. And you know, we sat there. And as soon as they announced the name of the shooter, she -- she just lost it. And what did she say to you? And what did you 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? Reporter: And while authorities here at ft. Hood were aware Lopez suffered from mental issues, seeing a psychiatrist last month. Today, in Washington, the secretary of the army said there were no warning signs of anything like this. No indication on the record of that examination that there was any sign of likely violence, either to himself or to others. Reporter: But what no one knew is as he was seeing that psychiatrist, Lopez had purchased a .45 caliber handgun, from this store, guns galore, on March 1st. The same gun store near ft. Hood, where major nidal Hasan bought his gun in 2009, before he killed 13 here. A community and its resiliency being tested all over again. When we were in that E.R. Today, the doctor treating the critically wounded, telling the three patients he was upgrading to serious condition tonight. So, some good news here at ft. Hood. That's right. And, David, you were telling us, as well, some lives were saved by the lessons learned five years ago at ft. Hood. Reporter: Yeah. Incredibly. You heard that alert in the piece there. And the tweet that went out immediately. We read that tweet when it went out. And you talk to anyone here. They tell you, they're convinced this could have been much worse had it not been for the immediate action of everyone here at ft. Hood, when that shooting began. Diane? David Muir, on the scene tonight.
This transcript has been automatically generated and may not be 100% accurate.