The US Air Force has said it is investigating its apparent failure to enter information about Texas gunman Devin Patrick Kelley's criminal history into the national database.

Ex-airman Kelley was court-martialled for domestic violence in 2012, and was barred from owning or buying guns.

But last year he was able to purchase a rifle he used in Sunday's attack on a small church outside San Antonio.

He killed 26 people and fled the scene. He was later found dead in his car.

Police said he died from what appeared to be a self-inflicted gunshot wound after being chased by armed bystanders.