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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.
Kelley was also shot in his leg and torso by a citizen, the regional director for the Texas Department of Public Safety, Freeman Martin, was quoted as saying by the Associated Press.
It said it was now - together with the Department of Defense - investigating the handling of Kelley's criminal records.
Texas Governor Greg Abbott earlier said Kelley, 26, should not legally have been allowed to own a firearm, after having been denied a gun owner's permit by the state.
Kelley bought the rifle from the Academy Sports Outdoors shop in San Antonio. The store issued a statement sending "deep condolences" to the victims' families.
Kelley was armed with the semi-automatic rifle and two pistols when he attacked the First Baptist Church of Sutherland Springs during Sunday services.
He had argued with his mother-in-law before the rampage, officials said.
Kelley called his father after he was shot by an armed bystander and said he did not think he would survive.