The man who slaughtered more than two dozen churchgoers in Texas forced a 13-year-old girl to surrender her virginity when he was 18.

Brittany Adcock, 22, says Devin Kelley was a "monster" who forced her to pose nude for him and asked her to move into his home where she would be his "topless maid" - even after he had married.

Kelley committed the deadliest mass shooting on record in Texas when he gunned down 26 people at the First Baptist Church in Sutherland Springs on Sunday (local time). The unborn child of a pregnant woman who was among the dead.

Devin Kelley, the suspect in the shooting at First Baptist Church in Sutherland Springs, Texas. Photo / AP
Devin Kelley, the suspect in the shooting at First Baptist Church in Sutherland Springs, Texas. Photo / AP

Another 20 people were wounded and half of them are still in critical condition.

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Adcock told the Daily Mirror she met Kelley at a shopping centre when she was just 13 years old and he was legally an adult.

Shortly after they met, their relationship turned sexual.

"It wasn't long after my 13th birthday but it didn't deter him despite him being a man," she said.

"Devin would beg to take pictures of me naked; to please him, I agreed.

"I was so naive. He was a lot older than me and I was bowled over an older guy would show interest in me.

"He took about a dozen pictures and then held them over me for the rest of his life."

Soon after they met, Kelley "took my virginity", she said.

"It was at my mother's house. It was just awful when I look back."

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Even after she broke off their relationship, Kelley continued to pursue her for years even though he had gotten married twice.

"One of the worst things was just after he married his second wife three years ago," she said.

Brittany Adcock was 'bowled over' an older man showed interested in her. Photo / Facebook
Brittany Adcock was 'bowled over' an older man showed interested in her. Photo / Facebook

"He said he would take care of me on condition that I live with them as his topless maid."

Adcock said that Kelley texted her a photo of a topless girl and asked if it was her.

She also claimed that Kelley created a fake Facebook profile, which he used as a cover to chat with her.

Officials said Kelley, 26, killed himself during a failed getaway attempt after he was wounded by an armed civilian.

Two handguns belonging to the killer also were recovered.

"He was a monster," Adcock said of Kelley. "The world is a better place now [that] he is gone.

"I now know he was a paedophile. He used me to fulfill his sick kicks."

Adcock said she thinks Kelley was a "ticking time bomb" immediately after he was kicked out of the Air Force in 2012.

Kelley escaped from a mental hospital in 2012 as he faced court-martial on domestic violence charges for which he was later convicted, a police report revealed.

The report also disclosed that police who were alerted to Kelley's escape were advised that he posed a "danger to himself and others" after being "caught sneaking firearms" onto the Air Force base in New Mexico where he was stationed.

Brittany Adcock says that Texas church gunman Devin Kelley (left) was a
Brittany Adcock says that Texas church gunman Devin Kelley (left) was a "monster" who forced her to pose nude for him. Photo / Facebook

The person who reported the escape, according to the report, warned that Kelley, then aged 21, had been "attempting to carry out death threats against his military commanders and suffered from mental disorders".

He was apprehended at an El Paso, Texas, bus station shortly after he had run off, according to a police report filed in that city.

Kelley's troubled Air Force background has been a focus of investigators in the tiny Texas town of Sutherland Springs since he stormed into a church with a semi-automatic assault rifle and opened fire on worshipers.

Where to get help:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

LIFELINE: 0800 543 354 (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
KIDSLINE: 0800 543 754 (available 24/7)
WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757