The US Air Force says it has launched a review after failing to provide information to authorities that would have stopped the Texas gunman from legally buying firearms.
26 people were killed when Devin Kelley opened fire at a church in Sutherland Springs on Sunday.
10 other people remain in a critical condition.
Detectives say the shooter had argued with his mother-in-law beforehand.
It has now emerged that, as an airman, Kelley was convicted five years ago of domestic assault against his wife and stepson.
The conviction should have prevented him from legally buying firearms.
The Air Force now says: "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."
It has launched a review, while the US Department of Defence also says it is investigating the matter.