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By Phil Helsel and Associated Press

The civilian head of the Air Force said Thursday that the military is trying to figure out why a domestic violence offense for the man suspected of killing 26 people at a Texas church wasn't reported to the FBI — and whether it's a systemic problem.

Devin Patrick Kelley's conviction at court-martial for domestic violence in 2012 should have prevented him from legally buying a firearm, officials have said.

"The offenses for which the shooter in Texas was court martialed should have been reported," Air Force Secretary Heather Wilson said at a Pentagon news conference Thursday. "That's why we launched a full-scale review of this case and all others like it."

Kelley, 26, of New Braunfels, Texas, opened fire outside and inside First Baptist Church of Sutherland Springs on Sunday, killing 26 people, including several children, authorities said.

Related: Texas Church Where 26 Were Massacred Could Soon Be Demolished

Kelley's name should have been entered in the National Criminal Information Center, Air Force spokeswoman Ann Stefanek said in a statement Monday, but the special investigations office at Holloman Air Force Base, New Mexico, failed to transmit the domestic violence charges.

Wilson said Thursday that a draft report on why Kelley's criminal history wasn't reported is expected to be completed next week. She said the Air Force has interviewed about 100 people associated with the matter since Sunday.

Wilson said a full review of Air Force databases is being conducted to determine whether the problem is systemic. She said one database goes back to 2002 and another to 1996.

A retired Air Force colonel who supervised prosecutors when Kelley was tried, said Kelley was convicted of fracturing his baby stepson's skull and assaulting his first wife, Tessa, at Holloman.

Related: Grandmother Died Shielding Grandson During Texas Church Shooting

He was confined for a year, given a bad conduct discharge and reduced in rank to E-1, or airman basic, Stefanek said.

Had Kelley's name been entered in the National Criminal Information Center database, it would have alerted whoever sold Kelley his weapons that he was ineligible to own firearms under the 1996 Lautenberg Amendment, which bars anyone convicted of domestic violence — even misdemeanors — from getting access to guns.

Security video from inside the sanctuary showed Kelley methodically shooting people in the head, a federal source close to the investigation told NBC News. Kelley died later of what appears to have been a self-inflicted gunshot wound.