Texas church gunman Devin Patrick Kelley was armed with three guns and had argued with his mother-in-law before the rampage, officials say.
The attack on the small church outside San Antonio during Sunday services left 26 people dead - half were children.
Kelley called his father after he was shot by an armed bystander and said he did not think he would survive.
US officials said he was not legally permitted to own the weapons, including a semi-automatic rifle and two pistols.
Freeman Martin, the regional director for the Texas Department of Public Safety, told reporters it appeared that Kelley died from a self-inflicted gunshot wound after first being shot by a "good Samaritan".
"This was not racially motivated, it wasn't over religious beliefs," Mr Martin said.
"There was a domestic situation going on with the family and in-laws," he said, adding that Kelley had sent threatening texts to his mother-in-law.
"We know that he expressed anger towards his mother-in-law, who attends this church," Mr Martin added.
The small town where everyone knows a victim
His mother-in-law, Michelle Schields, was not present during the shooting at the First Baptist Church of Sutherland Springs.
However, her mother, Lula White, was killed in the hail of bullets, according to relatives' Facebook posts and interviews with the New York Daily News.
His mother-in-law, Michelle Schields, was not present during the shooting at the First Baptist Church of Sutherland Springs.
However, her mother, Lula White, was killed in the hail of bullets, according to relatives' Facebook posts and interviews with the New York Daily News.
Who were the victims?
Wilson County Sheriff Joe Tackitt said between 12 and 14 children were among those killed in the attack in Sutherland Springs, a small town 30 miles (50km) south-east of the city of San Antonio.
The youngest of those who died was only a year old.
As many as eight members of the Holcombe family were killed in the shooting, the Washington Post reported.
The BBC spoke to the Holcombes' neighbour Pauline Garza, who said the family did not come back from church.
Sutherland Springs: The victims
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The First Baptist Church's pastor, Frank Pomeroy, told US media his 14-year-old daughter, Annabelle, was among those killed.
Officials said 23 people were found dead inside the church while two people were fatally shot outside. Another died in hospital, authorities said.
Of the 20 wounded, 10 are in hospital in a critical condition.
Who was the gunman?
Kelley was court-martialled in 2012 after he was accused of assault against his wife and step-son. He was sentenced to 12 months' confinement.
He received a "bad conduct" discharge two years later, according to Ann Stefanek, a US Air Force spokeswoman.
Who was Sutherland Springs gunman?
Texas Governor Greg Abbott said: "It's clear this is a person who had violent tendencies, who had some challenges, and someone who was a powder keg, seeming waiting to go off."
Mr Abbott added that the attacker should not legally have been allowed to own a firearm, after having been denied a gun owner's permit by the state.
