The gunman, identified as 26-year-old Devin Patrick Kelley, wearing black tactical-style gear walked into the church and started firing from an assault rifle just after the Sunday morning service began at the First Baptist Church in Sutherland Springs.
Texas law enforcement officials said that the shooting was not racially or religiously motivated, rather it was connected to a "domestic situation going on within [his extended] family, which wouldn't be a first for him".
But his mother-in-law was not inside the church when Kelley sprayed bullets on worshippers.
However, the gunman's grandmother-in-law was present at the church when Kelley stormed in from the backdoor, firing all the way to the front and firing on his way back out of the church, authorities said.
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He reloaded several times. He killed his grandmother-in- law during the attack, friends of the woman told the media.
"He was there to kill everybody. He is a mass killer of children and people ... he is a horrible monster," a source familiar with the matter told ABC News.
The ages of the deceased range from 18 months to 77 years, authorities said.
Eight members of a family, including a pregnant woman and a 17-month-old girl, were among 26 people killed in the massacre. The Holcombes lost family members across three generations.
Texas Governor Greg Abbott called the slaughter "the largest mass shooting" in the state's history and ordered flags across the state to fly at half-staff.
Evidence show that the suspect died from a self-inflicted gunshot wound, and multiple weapons were found in the vehicle, authorities said.
Kelley was previously a member of the US Air Force and served at Holloman Air Force Base in New Mexico from 2010 until his discharge.
He was court-martialled in 2012 for assault on his spouse and their child, according to Stefanek. He served a year in confinement and received a bad conduct discharge. His rank was also reduced.
Recently, he was fired from a brief stint as a water park security guard this summer, his former employer said.
His most recent job was as a night security guard at an RV park in New Braunfels. The manager said that he started working there six weeks ago, and he left early on Saturday, saying that he had a headache.
He never went to work on Sunday, the day of the shooting.