Gunman Devin Kelley, the former US serviceman who killed 26 churchgoers in Texas, had escaped from a mental hospital back in 2012.
At the time he had been facing a court-martial on domestic violence charges after assaulting his wife and stepson.
A police report has also disclosed that Kelley had later been arrested at a bus station and officers were told he was a danger to himself.
The assault charge should have legally barred him from owning guns. But the Air Force acknowledged it inexplicably failed to enter his conviction into a government database that all licensed firearms dealers are required to use to screen prospective gun buyers for their criminal history.
The Air Force will review domestic violence reporting after Texas church shooting https://t.co/TJpDyFbJlE— TIME (@TIME) November 8, 2017
Kelley’s troubled background has been a focus of investigators in Sutherland Springs since he stormed the church there with a semi-automatic rifle and opened fire on worshipers.
Kelley eventually killed himself during a failed gettaway attempt after he was wounded by an armed civilian.
The massacre which ranks as the deadliest mass shooting by a single gunman in Texas history has rekindled an ongoing debate over gun ownership.