The gunman who mowed down 26 worshippers in a Texan church on Sunday fled from a mental clinic in 2012 after being caught smuggling weapons on to a military base and issuing death threats to his commanding officers.

Devin Kelley was stationed at an Air Force base in New Mexico where he worked before facing a court-martial in 2012 for repeated assaults on his then wife and stepson. After his escape from the mental health facility he was detained by police, according to an investigation report seen by KPRC news in Houston.

Police advised that Kelley 'was a danger to himself and others as he had already been caught sneaking firearms' on to the base, the report states, adding that he 'was attempting to carry out death threats made on his military chain of command'.

It comes after the US Air Force said on Monday they had failed to enter Kelley's record of domestic violence into the national background checks for firearms sales. If they had done so red flags would have been raised when he tried to buy guns.

A motive for the attack is still unknown although on Monday law enforcement officials said there had been a 'domestic situation' with his mother-in-law, who was known to attend the church, where he threatened her via text.

One couple in the church who survived the shooting told KSAT local news that after the initial spray of gunfire he walked up and down the church shooting anyone who made a noise.

Rosanne Solis, who was sitting near the entrance, said Kelley burst into the church shouting: 'Everybody die!'

She said: 'Everybody was saying, 'Be quiet! It's him, it's him!'

She added: 'Everybody got down, crawling under wherever they could hide. He was shooting hard.'

Solis' husband, Joaquin Ramirez, told how he walked up and down the aisles, looking for survivors to shoot including young children who were crying who he shot at point blank range.

Police have been unable to unlock Kelley's phone to find more details of the attack and his motives and have sent it to FBI investigators to examine.