WILSON COUNTY, Texas -- Disturbing new details are emerging about the background of Devin Kelley, the New Braunfels man who opened fire on a small church congregation in Sutherland Springs, Texas killing 26 people.

Wilson County Sheriff Joe Tackitt says Kelley, 26, was dressed in black when he arrived at First Baptist Church in Sutherland Springs at about 11 a.m. Sunday. He was also wearing a ballistic vest and tactical gear while being armed with a Ruger assault-type rifle.

Twenty-six people were killed and 20 people were injured. Most of the deceased are children, the Tackitt said on Monday.

Kelley was found dead in Guadalupe County a short time later. It's not clear if he was shot by someone else or if his wound was self-inflicted.

Tackitt says Kelley's in-laws and and wife sometimes attended First Baptist, and officials believe the shooting was domestic-related. They were not there on Sunday, Tackitt said, but his mother-in-law says she received threatening text messages from Kelley sometime before the shooting.

Kelley, 26, received a bad conduct discharge and was court-martialed from the United States Air Force, where he served from 2010 to 2014, after he was convicted of assaulting his wife and child. He served one year in prison for the assault.

Social media posts that Kelley made are also being looked at -- one of which included a picture of an AR-15 semiautomatic weapon. Officials know he had purchased four guns -- two from Texas and two from Colorado.

Video from inside the church is being reviewed, officials said.

KENS 5 contributed to this report.