Days after the deadliest mass shooting in Texas history, the US Air Force acknowledged it did not report the shooter's violent criminal history to the FBI, as required by the Pentagon.

If his past offences had been properly shared, they would have prevented him from buying a gun.

Authorities have yet to release a specific motive on why 26-year-old Devin Kelley committed the shooting.

But they have revealed his mother-in-law attended the church and he had threatened her. That's why investigators say it was a "domestic situation," that he was on the hunt for his mother-in-law.

While police say the attack was neither racially nor religiously motivated, CBN News has reported Kelley was an atheist who was antagonistic to Christians.

After watching a video of the massacre inside this church, law enforcement officials say without a doubt the shooter intended to kill as many people as possible.

"There are many ways he could have taken care of the mother-in-law," said Texas DPS Trooper Freeman Martin. He added, "I think he came here with a mission. I think he came here with a purpose and a mission."

He killed 26 people ranging in age from 18 months to 77 years old.

Fourteen children in all were killed, including the pastor's 14-year-old daughter who was attending the service while her parents were out of town.

Once outside, police say Kelley was confronted by two men—one of them a former National Rifle Association instructor who used his gun and his training to stop Kelley.

"I just wish I could have gotten there faster, I'm no hero, I'm not," said Stephen Willeford who officials say stepped in to stop Kelley.

Willeford fired several shots that hit Kelley and then jumped into another neighbor's truck as the two of them chased Kelley for miles.

Willeford said, "Every time I heard a shot I knew that probably represented a life. I was scared to death."

An autopsy reveals Kelley was hit twice by Willeford, and a third wound was self-inflicted to the head.

Police say as Kelley tried to get away he used his cell phone and notified his father that he had been shot and didn't think he would make it.

Officials say Kelley did not have a license to carry firearms.

He purchased four weapons in total, in 2014, 2015, 2016, and 2017. Two were bought in Colorado, and two in Texas, ATF officials said.

Three weapons were recovered at the scene, a Ruger AR-556 rifle found at the church, and two handguns, a Glock 9mm and a Ruger 22, found in his car, according to Fred Milanowski, special agent in charge of the ATF Houston.

A law enforcement source tells CBN News that Kelley purchased the Ruger AR-556 from an Academy Sports and Outdoors location in San Antonio in April 2016, and one of the handguns, the Glock 9mm, at Specialty Sports in Colorado Springs in December 2014.

CBN News has learned the shooter was discharged from the US Air Force in 2014 after serving a one year sentence for assaulting his first wife and his 11-month-old stepson.

Military records show he hit the child in the head with enough force to produce death or bodily harm.

In theory the domestic abuse charges should have barred him from buying guns.

Now the US Air Force admitted it failed to pass along the information that would have prevented Kelley from obtaining the murder weapon.

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