The gunman who murdered 26 people at a Texas church Sunday is believed to have been driven by a "domestic" dispute, officials said Monday.

"This was not racially motivated, it wasn't over religious beliefs. There was a domestic situation going on with the family and in-laws," Freeman Martin, regional director of the Texas Department of Public Safety, told a news conference.

"The suspect's mother-in-law attended this church," he said, adding that 26-year-old shooter Devin Patrick Kelley had sent her "threatening texts" prior to the mass shooting.

"We know he sent threatening ... that she had received threatening text messages from him," Martin said.

Kelley's in-laws were not at First Baptist's church service Sunday, but did go there to talk to authorities Sunday afternoon.

"I heard that they attended church from time to time, not on a regular basis," Sheriff Joe Tackitt said earlier Monday.

Martin also there is video from inside the church, but is not prepared to speak about what it shows, at this time.

ATF agent Fred Milanowski said during a news conference Monday that officers recovered a Ruger AR-556 rifle at the church.

Milanowski said two additional handguns were recovered from the vehicle driven by Kelley — a Glock 9mm and a Ruger .22-caliber. Milanowski says all three weapons were purchased by the now-deceased suspect.

Kelley, 26, walked into the white-steepled First Baptist Church in rural Sutherland Springs carrying an assault rifle and wearing a skull mask and black tactical gear, then opened fire during a Sunday prayer service. He wounded at least 20 others, officials said.

Authorities say that evidence at the scene leads them to believe that Kelley died of a self-inflicted gunshot wound after he crashed his car. He had been chased by armed bystanders.

They say Kelley also used his cellphone to tell his father that he had been shot and didn't think he would survive.

Reuters contributed to this story.