The man responsible for the Texas shooting that killed 26 churchgoers in a rural church this week has been described as an “outcast” amid revelations he escaped from a mental hospital in 2012.

Devin Kelly, 26, is accused of calmly gunning down more than half the congregation at the First Baptist Church in Sutherland Springs on Sunday and wounding at least 20 others in a premeditated attack.

He died from what authorities believe was a self-inflicted gunshot wound after being chased from the church by two local residents.

Military officials and police in three US states are now piecing together his 10 years of threatening and abusive behaviour that culminated in the Texas shooting.

Kelley graduated from New Braunfels High School on the outskirts of San Antonmio in 2009, where former classmates described him as an “outcast”.

One former classmate, Kayla Shearer, described him on Facebook as “twisted”, according to The Independent.

“He ended up assaulting me. He would stalk me by repeatedly calling me — even prank calling me, saying really weird stuff,” Katy Landry told NBC News.

Schoolmates of Kelley have recounted his disturbing behaviour. Photo: Seth Poppel Yearbook Library

Kelley enlisted in the Air Force shortly after and was assigned to Holloman Air Force Base in New Mexico, with the responsibility of moving passengers, cargo and personal property in military transportation. He married for the first time in 2011, but that soon ended violently.

Kelley was convicted by a US Air Force court-martial of assaulting his wife and stepson while serving in the Air Force, according to a police report.

He was sent to a mental hospital in Santa Teresa, New Mexico after the court-martial and briefly escaped while stationed at the state’s Air Force base.

The police report states that authorities were warned Kelley could be a danger to himself and others after his escape, and that he had been caught sneaking weapons onto the Holloman base.

Kelley also attempted to “carry out death threats that he had made on his military chain of command”, the report read.

According to Air Force records, Kelley repeatedly struck, kicked and choked his first wife just months into their marriage, and hit his stepson’s head with “a force likely to produce death or grievous bodily harm”.

The 18-month-old suffered a fractured skull in the incident.

Kelley was also involved in a domestic dispute with the parents of his second wife, whom he married in 2014, and had sent threatening text messages to his mother-in-law before the shooting.

In February 2014, sheriff’s deputies were called one night to Kelley’s New Braunfels home to investigate a potential domestic violence case.

Comal County spokesman Paul Anthony cited a sheriff’s report as saying a friend of Kelley’s then girlfriend, Danielle Shields, told authorities she received a text message that indicated “Her [Danielle’s] boyfriend was abusing her”.

When sheriff’s deputies arrived, people at the home said there was a “misunderstanding” and no arrests were made.

Kelley and Ms Shields were married two months later.

In August 2014, while living in Colorado, Kelley was charged with  animal cruelty.

A neighbour told a deputy he chased a dog, jumped on top of it and struck it with a closed fist several times, the Denver Post quoted an incident report as saying.

The Denver Post reported that a protection order was issued against Kelley in January 2015.

Kelley later returned to Texas and took jobs as a security guard in New Braunfels.

He appeared to have targeted the First Baptist Church because his estranged wife’s family were part of the congregation.

The Texas shooting ranks as the fifth deadliest by a single gunman in US history.

The dead ranged from 18 months to 77 years old, and included the unborn child of a pregnant woman, who was killed. Ten of the injured are in a critical condition.

Air Force failed to report  conviction

The Air Force has admitted it failed to submit information about Kelley’s conviction to the FBI’s criminal information database, which is used by gun dealers to check prospective buyers for criminal backgrounds.

US media report the lapse could explain why Kelley was allowed to purchase guns in recent years, and exposed a previously unnoticed weak link in the system of background checks.

According to The Wall Street Journal, Kelley purchased one gun in 2016 and another this year at two different stores in San Antonio.

The FBI has also reported the agency has been unable to access an encrypted phone used by the gunman.

FBI official, citing encryption tech, says federal agents have not been able to access the Texas shooter's phone https://t.co/CacbcOGFxq

— NBC News (@NBCNews) November 7, 2017

FBI special agent Christopher Combs expressed his frustration with the technology industry at a press conference on Wednesday local, as the phone was transported to FBI headquarters in Virginia for examination.

“Unfortunately, at this point in time, we are unable to get into that phone.”

– with AAP