A man pulled out a rifle and opened fire during a live-streamed church service in Fort Worth, Texas, killing himself and one other person.
A security guard who attempted to protect the congregation during the Sunday morning service is thought to be his victim.
Video on West Freeway Church of Christ's YouTube channel, which has since been taken down, shows the full attack.
The gunman appears to walk in almost unnoticed, before opening fire and shooting someone. A guard stationed at the back of the room immediately draws his weapon, taking a single shot from a distance and taking down the gunman. People instantly scramble for cover, diving between the rows of benches laid out for the congregation.
Mike Tinius, who was at the church, told the New York Times that the gunman's victim was another security guard, and called him a dear friend.
"He was trying to do what he needed to do to protect the rest of us. It's extremely upsetting to see anyone committing violence."
Mr Tinius said he didn't know the gunman and that the shooting appeared to be random.
Isabel Arreola told CBS News: “It was the most scariest thing. You feel like your life is flashing before you. I was so worried about my little one."
A third person is being treated at a hospital and all of the victims are male. Authorities have not released any details about what led to the shooting.
In an interview with Dallas TV station KTVT, Mike Drivdahl, the spokesman for the Fort Worth Fire Department, said he believes the gunman was among the three who were shot.
Texas Attorney General Ken Paxton said in a tweet on Sunday that his office would assist in any way necessary.
"Please pray for any victims and their families, this congregation, and the law enforcement officials at the scene," Mr Paxton said.
