Authorities work to find a motive in the Texas mass shooting

The gunman was allegedly in a dispute with family members.
1:53 | 11/07/17

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Transcript for Authorities work to find a motive in the Texas mass shooting
We turn next, though, to the gunman. And the questions at this hour. Why was he allowed to obtain that arsenal. The image of a troubled young man now emerging this evening, discharged from the air force after a domestic assault conviction. And re cently posting this photo of a semiautomatic rifle on his Facebook page. Here's ABC's senior justice correspondent Pierre Thomas tonight. Reporter: Tonight, law enforcement officials have viewed the video of the massacre and one of them tells ABC news there's little doubt the gunman came to 2 church to kill everyone in that building, period, and called him a horrible monster. That stark assessment, as police today said the genesis of the killing spree was rooted in a domestic dispute. The suspect assess parentally hunting his mother-in-law. The suspect's mother-in-law attended this church. We know that she had received threatening texts from him. Reporter: Authorities say Kelley showed up with a bullet proof vest and a has wk a skull pictured on it. And the evidence is mounting that for Kelley, violence had been a family affair. He was discharged from the air force after serving a one-year sentence for assaulting his first wife and infant stepson. Military records show he hit the infant in the hard enough to produce death. They would be prohibited from possessing or purchasing firearms. Reporter: Tonight, the U.S. Air force admitting they failed to pass along the information that would have prevented Kelley from obtaining the murder weapon. Let's drill down on that Pierre. More on that late word from the military, they didn't forward information that would have prevented him from obtaining his weapons. A law enforcement official saying it was a administrative error? Reporter: Yes, David. The air force failed to pass along the information to the FBI. The sale of that murder weapon should have been blocked, David. Pierre Thomas with us tonight. Pierre, thank you.

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

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