By Monday night, investigators had learned that the shooter, 26-year-old Devin Kelley, shot himself in the head after opening fire during the Sunday service.
But before that, Kelley had a shootout with Stephen Willeford, a neighbor who lived near the church.
"The people of that church - they're friends of mine. They're family. And every time I heard a shot, I knew that probably represented a life," Willeford said.
VIDEO: Everything we know about Devin Kelley
Willeford said his daughter first alerted him a man was shooting at the church, and the former NRA instructor didn't hesitate. He grabbed his rifle from his safe and ran barefoot down the street, straight towards the gunfire.
"I was scared to death," Willeford said. "I was scared for me and I was scared for every one of them."
Willeford said he locked eyes with Kelley, who was wearing all black, a ballistic vest and a mask. The two fired at each other, and Kelley was hit in the torso and in the leg.
Kelley dropped his rifle and took off in his Ford Expedition. Willeford and another man chased him for six miles. Kelley crashed his SUV. Authorities found him dead. They say he called his father then shot himself in the head.
Why Kelley did this is still unclear.
If Willeford hadn't been there, investigators fear more people would have lost their lives.
"I'm no hero. I am not," he said. "I think my god, my Lord protected me and gave me the skills to do what needed to be done."
Investigators say Kelley had problems with his estranged wife who attended the church. In 2014, he was convicted of domestic abuse. Recently, authorities say Kelley sent his mother-in-law threatening text messages.
"We know there was conflict between the families. Whether that was revenge or not would be speculation on my part," said Freeman Martin with DPS.