SUTHERLAND SPRINGS, Texas -- A gunman dressed in black tactical-style gear and armed with an assault rifle opened fire inside a small South Texas church, killing 26 people in an attack that claimed tight-knit neighbors and multiple family members ranging in age from 18 months to 77-years-old.
*26 dead killed in the shooting - 12 to 14 of them children
*20 injured - 4 in serious condition - 10 in critical condition
*Suspect may have died from self-inflicted gunshot wound, investigation ongoing
*Incident not racially or religiously motivated
Authorities said the attacker, 26-year-old Devin Patrick Kelley, sent threatening text messages to his mother-in-law before the attack, which appeared to stem from a domestic situation.
Investigators have concluded that the deadliest mass shooting in state history was not racially or religiously motivated, Texas Department of Public Safety Regional Director Freeman Martin said.
Based on evidence at the scene, they believe that Devin Patrick Kelley died of a self-inflicted gunshot wound after he was chased by armed bystanders and crashed his car.
Authorities added that Kelley used his cell phone to tell his father that he had been shot and did not think he would survive.
The gunman's family relationships were uncertain. The sheriff said the shooter's former in-laws sometimes attended services at the church but were not there on Sunday. Martin said the text messages were sent to the gunman's mother-in-law, who attended the church. It was unclear if they were referring to the same people.
Once the shooting started Sunday at the First Baptist Church in Sutherland Springs, there was likely "no way" for congregants to escape, said Wilson County Sheriff Joe D. Tackitt Jr. Officials said about 20 others were wounded.
"He just walked down the center aisle, turned around and my understanding was shooting on his way back out," said Tackitt, who said the gunman also carried a handgun but that he didn't know if it was fired. Tackitt described the scene as "terrible."
"It's unbelievable to see children, men, and women, laying there. Defenseless people," he said.
During the news conference, officials said 23 of the 26 victims died in the church, while two died outside the church, and one died at the hospital.
RELATED: More about the victims in the Texas church shooting
Freeman Martin, a regional director of the Texas Department of Safety, said Monday that of the 20 injured, 10 people were still hospitalized in critical condition.
Martin said those treated in hospitals ranged in age from 5 to 73.
Tackitt told CNN on Monday morning that the gunman's former in-laws attended services at the church "from time to time" but weren't in attendance Sunday.
A U.S. official said Kelley lived in a San Antonio suburb and didn't appear to be linked to organized terrorist groups. Investigators were looking at social media posts Kelley made in the days before the attack, including one that appeared to show an AR-15 semiautomatic weapon.
Officials said he didn't have a license to carry a gun.
Kelley received a bad conduct discharge from the Air Force for assaulting his spouse and child and was sentenced to 12 months' confinement after a 2012 court-martial. Kelley served in Logistics Readiness at Holloman Air Force Base in New Mexico from 2010 until his 2014 discharge, Air Force spokeswoman Ann Stefanek said.
At the news conference, the attacker was described only as a white man in his 20s who was wearing black tactical gear and a ballistic vest when he pulled into a gas station across from the church, about 30 miles (48.28 kilometers) southeast of San Antonio, around 11:20 a.m.
RELATED: Texas church shooting: What we know about alleged gunman
The gunman crossed the street and started firing the rifle at the church, said Freeman Martin, a regional director of the Texas Department of Safety, then continued firing after entering the white wood-frame building, where an 11 a.m. service was scheduled.
As he left, the shooter was confronted by an armed resident who "grabbed his rifle and engaged that suspect," Martin said. A short time later, the suspect was found dead in his vehicle at the county line.
Federal agents, including ATF investigators and the FBI's evidence collection team, swarmed the small rural community of just hundreds of residents.
RELATED: Trump says Texas church mass shooting not a 'guns situation'
Several weapons were found inside the vehicle and Martin said it was unclear if the attacker died of a self-inflicted wound or if he was shot by the resident who confronted him. He said investigators weren't ready to discuss a possible motive.
Martin said 23 of the dead were found in the church, two were found outside and one died after being taken to a hospital.
The man who confronted Kelley had help from another local resident, Johnnie Langendorff, who told KSAT TV that he was driving past the church as the shooting happened. He didn't identify the armed resident but said the man exchanged gunfire with the gunman, then asked to get in Langendorff's truck and the pair followed as the gunman drove away.
Langendorff said the gunman eventually lost control of his vehicle and crashed. He said the other man walked up to the vehicle with his gun drawn and the suspect did not move. He stayed there for at least five minutes until police arrived.
"I was strictly just acting on what's the right thing to do," Langendorff said.
Among those killed was the church pastor's 14-year-old daughter, Annabelle Pomeroy. Pastor Frank Pomeroy and his wife, Sherri, were both out of town when the attack occurred, Sherri Pomeroy wrote in a text message.
"We lost our 14-year-old daughter today and many friends," she wrote. "Neither of us has made it back into town yet to personally see the devastation. I am at the Charlotte airport trying to get home as soon as I can."
Church member Nick Uhlig, 34, who wasn't at Sunday's service, told the AP that his cousin, who was 8 months pregnant, and her in-laws were among those killed. He later told the Houston Chronicle that three of his cousin's children also were slain.
President Donald Trump, who was in Japan, called the shooting an "act of evil," later calling the gunman "a very deranged individual."
Sunday evening, two sheriff's vans were parked outside the gate of a cattle fence surrounding the address listed for Kelley on the rural, western outskirts of New Braunfels, north of San Antonio.
Ryan Albers, 16, who lives across the road, said he heard intensifying gunfire coming from that direction in recent days.
"It was definitely not just a shotgun or someone hunting," Albers said. "It was someone using automatic weapon fire."
The church has posted videos of its Sunday services on a YouTube channel, raising the possibility that the shooting was captured on video.
In a video of its Oct. 8 service, a congregant who spoke and read Scripture pointed to the Oct. 1 Las Vegas shooting a week earlier as evidence of the "wicked nature" of man. That shooting left 58 dead and more than 500 injured.
Gov. Greg Abbott called Sunday's attack the worst mass shooting in Texas history. It came on the eighth anniversary of a shooting at the Texas' Fort Hood, where 13 people were killed and 31 others wounded by a former U.S. Army major.
The previous deadliest mass shooting in Texas had been a 1991 attack in Killeen, when a mentally disturbed man crashed his pickup truck through a restaurant window at lunchtime and started shooting people, killing 23 and injuring more than 20 others.
The University of Texas was the site of one of the most infamous mass shootings in modern American history, when U.S. Marine sniper Charles Whitman climbed the Austin campus' clock tower in 1966 and began firing on stunned people below, killing 13 and wounding nearly three dozen others. He had killed his wife and mother before heading to the tower, one victim died a week later and medical examiners eventually attributed the 17th death to Whitman in 2001.
RELATED: Worst mass shootings in US history
The Associated Press contributed to this post.
Police: Texas church gunman sent threatening texts to mother-in-law
More TOP STORIES News