Texas shooter had issues with mother-in-law who attended the attacked church

The gunman who killed 26 people at a church in the United States state of Texas had a history of domestic violence and sent threatening text messages to his mother-in-law, according to authorities which said the shooting was not racially or religiously motivated.

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'.

"We know that he expressed anger towards his mother-in-law, who attends this church," Freeman Martin of the Texas Department of Public Safety said, adding that the court-martialled former US Air Force airman had a history of sending her threatening texts.

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.

He reloaded several times. He killed his grandmother-in-law during the attack, friends of the woman told the media.

Authorities, however, also believe that the shooting was not a case in which suspect went to the church looking for his relatives but ended up killing many others.

"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.

Among the dead were 12 to 14 children. The church's pastor Frank Pomeroy's 14-year-old daughter, Annabelle, was also among the dead.

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.

He also ran into trouble with the law as a civilian, according to court records, which indicate he was cited for misdemeanor cruelty to animals in August 2014.

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. Kelley also used his cellphone to tell his father that he had been shot and did not think he would survive, authorities said.

IMAGE: Devin Kelley, 26, is shown in this undated Texas Department of Safety driver licence photo. Photograph: Texas Department of Safety/Handout via Reuters