Devin Patrick Kelley, the 26-year-old suspect who opened fire inside a South Texas church, leaving 26 dead, was an atheist whose ex-wife and former in-laws attended the church he targeted.

Kelley, who died shortly after the attack on the church Sunday, was dressed in all black and tactical gear when he opened fire at the First Baptist Church in Sutherland Springs, leaving over two dozen people dead and 20 others injured.

As authorities continue to search for an indication of what motivated the deadly attack on the congregants, disturbing information continues to emerge about Kelley.

Kelley, a former U.S. Air Force airman, was court-martialed and sentenced to a year in military prison for assaulting his wife and child in 2012, Air Force spokeswoman Ann Stefanek told The Washington Post. He received a bad conduct discharge, and reduction in rank and confinement for 12 months.

Kelley's first wife divorced him in 2012 in New Mexico, and he remarried in Texas in 2014. Authorities said there was a "domestic situation" with him and his in-laws, who attended the First Baptist Church from time to time, although they were not present during the attack. Shortly before the massacre, Kelley's mother-in-law had received threatening messages from him.

"We know that his ex- in-laws or in-laws came to church here from time to time," Wilson County Sheriff Joe Tackitt told CNN Monday. "They were not here yesterday. So we don't know why he actually showed up yesterday."

His animosity toward Christians was no secret; Nina Rose Nava, who went to school with Kelley, wrote on Facebook how he would dismiss Christians as "stupid."

"In in complete shock! I legit just deleted him off my fb 'cause I couldn't stand his post. He was always talking about how people who believe in God we're stupid and trying to preach his atheism," Nava wrote.

Still, authorities revealed Kelley did some work as a bible teacher, teaching young children at First Baptist Church Kingsville's Vacation Bible School. The church confirmed his participation in a statement.

"Media reports have made a connection between Devin Patrick Kelley and First Baptist Church of Kingsville, Texas. According to our records, Kelley volunteered one night as a helper during the 2014 Vacation Bible School. He was not a member of the First Baptist Church of Kingsville, nor did he serve in any other capacity," the church said. "Our congregation would like to offer our prayers and deepest condolences to the Sutherland Springs community mourning the loss of their loved ones."

Kelley, who carried out the attack with a Ruger AR-556 rifle, had tried to get a license to carry a gun in Texas but was denied by the state, Texas Gov. Greg Abbott said, citing the director of Texas' Department of Public Safety.

"So how was it that he was able to get a gun? By all the facts that we seem to know, he was not supposed to have access to a gun," Abbott said. "So how did this happen?"

The massacre, which has been identified as the deadliest mass shooting in Texas history, left about 4% of the small town's population dead. The slain victims ranged in age from 18 months to 77 years old and include 14-year-old Annabelle Pomeroy, daughter of church pastor Frank Pomeroy.

"We lost more than Belle yesterday, and the one thing that gives me a sliver of encouragement is the fact that Belle was surrounded by her church family that she loved fiercely, and vice versa," Sherri Pomeroy said.

"Now most of our church family is gone. Our building is probably beyond repair. ... As senseless as this tragedy was, our sweet Belle would not have been able to deal with losing so much family."

Twenty-three people died inside the church, two died outside the church, and one at the hospital, according to CNN.

On Sunday evening, residents of Sutherland Springs held a vigil for the victims of the massacre. 

Gloria Rodriguez Ximenez, who attended the vigil, described her town as a place where "everybody knows everybody."

"This is a small, Christian town, a very small community," she said. "Everybody's united. Everybody's so close to everybody."

She added, "I can feel the pain everybody's going through. There's so much hurt for a small town."