Devin Patrick Kelley posted a photo of a semiautomatic rifle on Facebook alongside the caption "she's a bad b---h" a week before the shooting.
Kelley, 26, walked into a church in a small town near San Antonio on Sunday and left at least 26 people between the ages of five and 72 dead and wounded 20 more.
A law enforcement official close to the investigation told The Daily Beast that the rifle Kelley used on Sunday is similar to the one in his Facebook post, but could not confirm it is the same weapon.
The weekly service at Sutherland Springs First Baptist Church started at 11 a.m., with the congregation inside the building awaiting a sermon when Kelley arrived. Police say he wore a ballistic vest.
Kelley crossed the street in his vehicle, got out and began firing using a Ruger AR assault-type rifle. He then moved to the right side of the church, continuing to fire and then into the building — still firing bullets — before later being found dead in a neighbouring county.
Officials say Kelley lived in a San Antonio suburb and does not appear to have any ties to organized terrorism groups. Investigators are looking at social media posts Kelley may have made in the days leading up to the mass shooting, including one that appeared to show an AR-15 style semi-automatic weapon.
Neighbors of an address listed in Kelley's name said they heard intense gunfire from the direction of the house the past couple days.
"It's really loud," said Ryan Alberts, 16, who lives across the road. "At first I thought someone was blasting. It was someone using automatic weapon fire."
Kelley served in logistics at Holloman Air Force Base in New Mexico from 2010 before being court-martialed in 2012 and dishonourably discharged two years later. Spokeswoman Ann Stefanek said Kelley was court-martialed on one count of assaulting his wife and another count of assault on their child. He received a bad conduct discharge, 12 months confinement and a reduction in rank.