The US Air Force has launched a review after failing to provide information to authorities that would have stopped the Texas shooter from legally buying firearms.
Devin Kelley killed 26 people and injured 20 others when he opened fire in the First Baptist Church in the town of Sutherland Springs on Sunday.
But five years earlier, as an airman, he was convicted by a court-martial on two charges of domestic assault against his wife and stepson.
He had hit the child "on the head and body with a force likely to produce death or grievous bodily harm" in 2011, as well as striking the child on other instances. He had also kicked, choked and struck his wife.
As well as being convicted and sentenced to 12 months' confinement, he received a "bad conduct" discharge from the military in 2014.
Pentagon rules say the Air Force should have reported Kelley's criminal history so it could be provided to the National Criminal Information Center Database, which is maintained by the FBI.
The nature of his conviction would have prevented him from buying or possessing firearms.
But federal officials have said the Air Force failed to report the details.
Kelley bought guns twice in San Antonio without raising any red flags. He also bought two guns in Colorado.
A rifle was found at the church, where Kelley's victims range in age from 18 months to 77 years old. Two handguns were found in his car. He died after being shot three times – twice by a citizen and the final time by his own hand.
Air Force spokeswoman Ann Stefanek said: "Initial information indicates that (Devin) Kelley's domestic violence offence was not entered into the National Criminal Information Center database by the Holloman Air Force Base Office of Special Investigations.
"The Air Force has launched a review of how the service handled the criminal records of former Airman Devin P. Kelley following his 2012 domestic violence conviction."
In a statement, the Department of Defense said: "Following yesterday's tragic events in Texas, the Department of Defense has requested the DoD Inspector General in concert with the Air Force to review the handling of criminal records in the case of former Airman Devin P. Kelley after his 2012 domestic violence convictions."
There would also be an investigation "to determine whether information about Kelley's conviction was properly entered into the National Criminal Information Center database".
Policies and procedures would also be reviewed to make sure records from other cases had been reported correctly, they added.
Police believe Kelley's killings were motivated by the domestic situation within his family and in-laws, and the motive was not religious or racial.