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.
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.
Kelley fled the scene after exchanging gunfire with a local resident who heard gunshots coming from across the road, Mr Martin said on Monday.
Johnnie Langendorff told local media that he picked up the Texas man, identified by US media as Stephen Willeford, and the pair pursued Kelley in a car chase.
He said they drove at speeds of up to 95mph (153km/h) until the gunman lost control of his car and crashed.
Kelley was later found dead inside his vehicle with what appeared to be a self-inflicted gunshot wound, police said.
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.
The First Baptist Church’s pastor, Frank Pomeroy, told US media his 14-year-old daughter, Annabelle, was among those killed.