Twenty-six people were killed and 20 were injured in Sunday's mass shooting at First Baptist Church in Sutherland Springs, Texas

Authorities believe it was “a domestic situation” — and not terrorism — that fueled Sunday’s mass shooting inside a small church in rural Texas, killing at least 26 people, including an 18-month-old child.

During a Monday press conference, an FBI official confirmed that the shooter, 26-year-old Devin Patrick Kelley, had “expressed anger towards his mother-in-law,” who was a member of the congregation at the First Baptist Church in Sutherland Springs, the site of Sunday’s attack.

The shooter, officials said, had sent his mother-in-law several threatening text messages, but no report was ever filed with police.

Kelley is a former Air Force member who dressed in all-black tactical gear and was wearing a ballistics vest when he began firing from outside the church at around 11:30 a.m. local time, during a service.

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Sutherland Springs, with a population of just over 700, is about 35 miles southeast of San Antonio.

After shooting from outside the church, the gunman then entered the church and continued to shoot. When the gunman exited the church, a local resident engaged him in gunfire, authorities said. The suspect dropped his weapon and fled, with the local resident in pursuit.

Credit: Darren Abate/AP

A brief chase ensued into neighboring Guadalupe County, Texas, where the shooter was later found dead in his vehicle. He was likely killed by a self-inflicted gunshot wound, authorities said Monday.

20 Injured, and 10 Still in Critical Condition

The shooting killed 26 — 23 inside the church, and two outside. One person died upon arriving at the hospital. A total of 20 were wounded; 10 remain in critical condition while 4 are listed as serious with 6 others in stable condition.

The youngest victim was only 18-months-old, with the oldest aged 77. None of the victims’ names have been released by officials.

Freeman Martin, the regional director of the Texas Department of Public Safety, said during Monday’s press conference that the gunman reached out to his father following the massacre, as he was being pursued by police.

Credit: Nick Wagner/Austin American-Statesman/AP

“The suspect used his cell phone to notify his father that he had been shot and didn’t think he was going to make it,” Martin said. “Subsequently, he shot himself … and the pathologist will determine the cause and manner of his death.”

An Air Force spokeswoman tells PEOPLE the shooter served at Holloman Air Force Base in New Mexico, working in logistics readiness. He was stationed there from 2010 until he was discharged in 2014, following a 2012 court-martial on accusations he assaulted his spouse and their child, the spokeswoman says.

The shooter received a bad conduct discharge and 12 months’ confinement as well as a reduction in rank, the spokeswoman says.

Authorities Investigating How Shooter Purchased Guns

Martin confirmed that because of the court-martial, the gunman “did not have a license to carry” the rifle and two handguns investigators recovered from the crime scene.

ATF Houston Special Agent in Charge Fred Milanowski said the shooter purchased two of the guns in Colorado and one in Texas.

“He did have a noncommissioned unarmed private security license similar to a security guard at a concert,” Martin explained, which was granted after he passed both a criminal history and background check. Authorities are trying to determine how the gunman was able to purchase the weapons.

Martin also said there is surveillance video of the shooting, which was recorded from inside the church.

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“The crime scene investigation will go on for days,” Martin said. “The video will assist in the investigation and it was a horrific event and at this time we’re not prepared to talk about the video.”

The gunman arrived at the church wearing a black mask with a white skull on it.

Officials say they plan to update the media later Monday evening.