By Holly Yan and AnneClaire Stapleton, CNN

(CNN) -- The gunman behind the Texas church massacre killed his grandmother-in-law during the attack, multiple friends of the woman told CNN.

Devin Patrick Kelley had domestic problems and texted his mother-in-law as recently as Sunday morning, not long before he carried out the largest mass shooting in Texas history, authorities said.

"We know that he expressed anger towards his mother-in-law, who attends this church," said Freeman Martin of the Texas Department of Public Safety. He said Kelley had a history of sending her threatening texts.

But his mother-in-law was not inside the First Baptist Church in Sutherland Springs when Kelley sprayed the congregation with gunfire and killed 26 people. The youngest killed was a 17-month-old girl, her family told CNN. The oldest was 77 years old, Martin said.

The gunman's grandmother-in-law, Lula White, was the grandmother of Kelley's wife and often volunteered at the church, according to friends and her Facebook profile.

The friends asked not to be named as the family has decided to not speak to the media about White's death at this time, though a few have posted on social media

Texas Gov. Greg Abbott called the massacre "the largest mass shooting" in the state's history.

Latest developments

Before the shooting, authorities say Kelley looked suspicious at a nearby gas station. "He was obviously suspicious to others based on the way he was dressed in all black," Martin said. He said Kelley also had a black mask with a white, skull-type face on it, but it was unclear whether he was wearing the mask at the gas station. Kelley was wearing a ballistic vest with a plate on the front, Martin said.

Ten of the 20 people wounded remain in critical condition Monday, authorities said. And virtually no one at the church was left unscathed, Wilson County Sheriff Joe Tackitt said. "I think nearly everyone had some type of injury," the sheriff told reporters Monday.

After Kelley left the church, he was confronted and shot by an armed resident. After Kelley was shot, he called his father, telling him he didn't think he was going to make it, Martin said. That's when Kelley shot himself.

It was not immediately clear whether Kelley died from the self-inflicted gunshot wound or from the shot by the resident. "However, investigators found evidence at the scene that indicates the subject may have died from a self-inflicted gunshot wound," Martin said.

Investigators have reviewed video footage from inside the church, Martin said.

Who were the victims?

The pastor of the church was out of town when the shooting occurred, but his daughter, 14-year-old Annabelle Pomeroy, was at the Sunday morning service and was killed, her mother Sherri Pomeroy said.

"We lost more than Belle yesterday, and the one thing that gives me a sliver of encouragement is the fact that Belle was surrounded by her church family that she loved fiercely, and vice versa," Sherri Pomeroy said.

"Now most of our church family is gone. Our building is probably beyond repair. ... As senseless as this tragedy was, our sweet Belle would not have been able to deal with losing so much family."

The church's visiting pastor was also among the dead, Sheriff Tackitt said.

So were eight members of one family, a relative and a community leader said. Those relatives span three generations and include a pregnant woman and three of her children.

Many of the victims have not been publicly identified, pending notification of family members.

Who was the shooter?

Kelley, 26, was a member of the US Air Force and served at Holloman Air Force Base in New Mexico from 2010 until his discharge, according to Air Force spokeswoman Ann Stefanek.

He was court-martialed in 2012 for assault on his spouse and assault on their child, according to Stefanek. He served a year in confinement, received a bad conduct discharge and had his rank reduced, she said.

In April 2016, Kelley purchased the Ruger AR-556 rifle he allegedly used in the shooting from a store in San Antonio, Texas, a law enforcement official said. There was no disqualifying information in the background check conducted as required for the purchase, a law enforcement official told CNN.

At one point, the shooter tried to get a license to carry a gun in Texas but was denied by the state, Abbott said, citing the director of Texas' Department of Public Safety.

"So how was it that he was able to get a gun? By all the facts that we seem to know, he was not supposed to have access to a gun," Abbott said. "So how did this happen?"

How the attack unfolded

The gunman was first spotted at a Valero gas station across the street from the church at about 11:20 a.m. (12:20 p.m. ET).

He drove across the street, got out and began firing even before he entered the church, Martin said.

David Flores told CNN that his father saw the shooter.

"My dad saw the gunman run into the church building and then he heard shots and saw people running," Flores told CNN. "People covered in blood and screaming. It was pandemonium everywhere."

As the shooter left the church, a local resident used his own rifle to engage him, Martin said. The gunman dropped his weapon and fled.

Johnnie Langendorff said he was driving to his girlfriend's house when he saw the shootout between the shooter and the armed resident.

The gunman took off in a Ford Explorer, Langendorff told CNN affiliate KSAT.

The resident "briefed me quickly on what had just happened and said we had to get him, and so that's what I did," Langendorff said.

They gave chase in his truck and called police. As they sped after him, Langendorff said, the shooter "eventually lost control on his own and went off into the ditch" in neighboring Guadalupe County.

"The gentleman that was with me got out and rested his rifle on my hood and kept it aimed at him [shooter], telling him to get out. There was no movement, there was none of that. I just know his brake lights were going on and off, so he might've been unconscious from the crash or something like that. I'm not sure."

Law enforcement later found the suspect dead of a gunshot wound inside his vehicle.

A law enforcement official briefed on the investigation said multiple weapons were found in the shooter's car. The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are involved in the investigation.

The grief and support

Hours after the shooting, residents of Sutherland Springs hugged one another, held candles and sang hymns in a vigil Sunday night. Abbott attended the event, in which faith leaders offered words of solace and prayers.

Authorities said 4% of the small town's population was killed in the shooting.

Sutherland Springs is the kind of place where "everybody knows everybody," said Gloria Rodriguez Ximenez, who attended the vigil.

"This is a small, Christian town, a very small community," she said. "Everybody's united. Everybody's so close to everybody."

She knows the First Baptist Church's pastor and his family, including their daughter who died.

"I can feel the pain everybody's going through. There's so much hurt for a small town," Ximenez said.

Others echoed the feeling of shock and heartache.

"My heart is broken," Wilson County Commissioner Albert Gamez Jr. told CNN. "We never think where it can happen, and it does happen. It doesn't matter where you're at. In a small community, real quiet and everything, and look at this."

Twenty-three people died inside the church, which has a small sanctuary with wooden pews and red carpeting. Two died outside the church, and one at the hospital.

The wounded are being treated at three hospitals including the Connally Memorial Medical Center in Floresville, University Hospital and Brooke Army Medical Center in San Antonio.