Investigators said Sunday's church massacre in Texas occurred amid an ongoing "domestic situation" involving the gunman and his relatives, at least one of whom had attended the church.

Authorities have not publicly identified a motive for the attack, but they emphasised Monday that the shooting did not appear to be fuelled by racial or religious issues.

They said the gunman's mother-in-law had attended the church but was not there Sunday, and that the shooter had sent "threatening texts" as part of the family dispute.

"This was not racially motivated, it wasn't over religious beliefs," Freeman Martin, of the Texas Department of Public Safety, said at a news briefing.

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"There was a domestic situation going on within the family and the in-laws."

Investigators have scoured the gunman's background since he opened fire Sunday morning on the pews of the First Baptist Church outside San Antonio, searching for a possible motive as the stories of those massacred began to emerge, the Washington Post reports.

There were 26 people killed in the church attack, the latest mass attack in a seemingly safe public space.

The dead included eight relatives spanning three generations in a single family. Officials had said the victims ranged in age from 5 to 72, but one family said a 1-year-old girl was also killed, one of the eight family members slain in the attack.

There were also 20 people wounded at the church, 10 of whom remained in critical condition Monday, Martin said.

Texas officials early Monday identified the attacker as Devin Patrick Kelley of New Braunfels, about 55km north of Sutherland Springs. They said the former Air Force member fired upon the churchgoers with a Ruger assault-style rifle before coming under fire from a local man.

Devin Patrick Kelley
Devin Patrick Kelley

Kelley fled and was chased, eventually running off of the roadway. He apparently took his own life, officials said.

Kelley, 26, was court-martialed in 2012 and sentenced to a year in military prison for assaulting his spouse and child, making him the latest mass attacker or suspect with domestic violence in his past. He was reduced in rank and released with a bad-conduct discharge in 2014.

Although few clues were initially offered about what made Kelley target a church in tiny Sutherland Springs, officials had suggested family ties could have played a role.

Wilson County sheriff Joe D. Tackitt jnr said Monday that Kelley's in-laws had attended the church but were not present Sunday. They came to the scene after the shooting.

Texas Govenor Greg Abbott suggested Monday morning that "there may have been a reason why this particular location was targeted", although he did not go into detail, saying that the details could emerge in the coming hours or days.

"I don't think this is a random act of shooting, a randomly chosen location, but obviously someone who is very deranged," Abbott said on the Today show.

Investigators are also probing how the attacker obtained his gun, noting that he had sought and failed to obtain a permit allowing him to carry a concealed weapon.

"By all of the facts that we seem to know, he was not supposed to have access to a gun, so how did this happen?" Abbott said in an interview on CNN.

"We are in search of answers to these questions."

A man wipes his eyes after a deadly shooting
A man wipes his eyes after a deadly shooting

Kelley worked briefly over the summer as an unarmed night security guard at a Schlitterbahn water park in New Braunfels, the company said.

The company also said that Kelley passed a Texas Department of Public Safety criminal background check before beginning work there. A spokeswoman said that Kelley was fired in July as the season was reaching its peak because he was "not a good fit".

The attack on Sunday left a staggering hole in a Texas town of fewer than 700 people about 50km southeast of San Antonio.

"Nearly everyone [inside] had some type of injury," Tackitt said. "I knew several people in there. It hasn't really hit yet, but it will."

Tackitt said the dead were all over the inside of the church. All of the bodies were removed overnight, Tackitt said, and were en route or already at the Bexar County Medical Examiner's office in San Antonio.

Inside the church, which remained cordoned off, was "a horrific sight", Tackitt said, adding "you don't expect to walk into church and find mauled bodies". Between 12 and 14 of the people killed or injured in the attack were children, he said.

Carrie Matula embraces a woman after the fatal shooting.
Carrie Matula embraces a woman after the fatal shooting.

For some, the church massacre also reinforced a sense of unease that no place could be considered immune from possible violence after a concert ground in Las Vegas, a Walmart in Colorado, a Nashville church and a bike path in New York all became scenes of death and bloodshed over the past six weeks.

President Trump appeared to try to preemptively steer the debate away from gun control after the slayings. At a news conference in Tokyo, Trump said he thought "mental health" was a possible motive, adding that it appeared the shooter was "a very deranged individual, a lot of problems for a long period of time". He did not provide further explanation.

The massacre "isn't a guns situation". Trump said. Referencing the armed civilian who apparently exchanged shots with the attacker, Trump said: "Fortunately someone else had a gun that was shooting in the opposite direction" or the rampage "would have been much worse".

No one inside the church was armed at the time of the attack, the sheriff said Monday, saying he was not surprised by that.

"People from this community would never think this could happen," he said.

Trump's reaction contrasted with his unrestrained calls for a death sentence for the Uzbek immigrant accused of killing eight people in an apparently Islamic State-inspired attack in Lower Manhattan last week.

Witnesses said the gunman in Texas, dressed in all black and wearing a tactical vest, began firing an assault rifle as he approached the church. Police said the gunman killed two people outside before entering the church and spraying bullets at the congregation during morning worship.

After the exchange of gunfire with an armed civilian, the gunman drove away with two men in pursuit.

It was "act now, ask questions later", said one of the pursuers, Johnnie Langendorff. By the time they caught up with him, however, the fleeing man had crashed his SUV into a ditch.

"He might have been unconscious from the crash or something like that, I'm not sure," Langendorff told reporters.

Tackitt, speaking to CBS News, said that after the gunfire and chase, the attacker was found dead. "At this time we believe that he had a self-inflicted gunshot wound, after he wrecked out."

The attack targeted young and old, tearing apart families.

Joe and Claryce Holcombe lost children, grandchildren and great-grandchildren all at once, a total of eight extended family members, the couple said in a phone interview with the Post.

Their son, Bryan Holcombe, 60, and his wife, Karla Holcombe, 58, were killed. Bryan was associate pastor for the church and walking to preach at the pulpit when he was shot, Joe Holcombe told the Post.

Among the dead was also their granddaughter-in-law, Crystal Holcombe, who was pregnant. She died with her unborn child and three of her children, Emily, Megan and Greg, according to Joe Holcombe. She had been at church with her husband, John Holcombe, who survived with two of her other children.

Their grandson, Marc Daniel Holcombe, and his infant daughter, who was about a year old, also died, Joe and Claryce Holcombe said.

Kevin Jordan, 30, was changing the oil in his Ford Focus ahead of a family road trip when he heard the pops of gunfire. When he stood up and turned his head, he saw a man wearing body armour, a vest and a mask walk down the sidewalk towards the church about 45m from his home.

"He was just spraying at the front of the church," Jordan said. "He was shooting outside at first, and then he walked to the door and started shooting inside."

After spotting the shooter, Jordan said, he ran inside his home, scooped up his son, alerted his wife and rushed his family into their bathroom, where they crouched and hid while calling 911. He said the shooter spotted him as he fled and took a shot that went through his front window, nearly hitting his 2-year-old son.

"I looked at the shooter, and he looked right at me," he said. When the shooting stopped, Jordan, who works as a medical assistant, ran to the church, hoping to help.

"I walked inside and just walked out. I couldn't handle it," he said. "It was bad. A lot of blood and bodies. The pews were knocked over. I'm a medical assistant and medical assisting does not prepare you for this."

Tucked a few hundred yards off Highway 87 amid scrubby farmland, the dusty and usually quiet streets of Sutherland Springs are lined with modest one-storey family homes and trailers. A town with few streetlights that typically goes dark after sundown flashed red and blue with police lights on almost every block.

In a matter of minutes, Sutherland Springs was transformed into the latest community riven by grief after a mass attack.

In late September, a masked gunman stormed into a small community church outside Nashville and shot seven people, including the pastor, killing one. Authorities said the suspect in that shooting, Emanuel Kidega Samson, might have been motivated by a desire for revenge for a 2015 shooting that targeted black churchgoers in Charleston - an attack that killed nine people.

The Texas attack also came just over a month after 58 people were killed at a Las Vegas country music festival, in what was the deadliest mass shooting in recent US history.

The gunman in Las Vegas, Stephen Paddock, killed himself after a lengthy shooting spree from his 32nd-floor Mandalay Bay hotel suite, and authorities have still not determined a motive.

Frank Pomeroy, the pastor of First Baptist Church, told ABC News that he was not present during the church service but that his teenage daughter, Annabelle Pomeroy, 14, was among the dead.

"She was very quiet, shy, always smiling, and helpful to all," Cynthia Rangel, 50, a resident of nearby Stockdale, said of the teenager. Rangel, a local emergency medical technician, said she knew three individuals who were hospitalised after the shooting and were undergoing surgery.

"This just all seems like it's not real."