Instead, Mr. Pomeroy was attending a class in Oklahoma City on the morning of Nov. 5. A three-word text message came across his cellphone. “Shooting at church,” it said.

He thought the sender, who was the church’s videographer, was kidding. “I hope you are joking,” he wrote back.

The reply came seconds later: “No.”

Mr. Pomeroy frantically tried to call parishioners who were at the service, but no one picked up. “By then, it was too late,” he recalled. “They had been shot.” He finally reached a friend, who was 10 minutes away from the church. The friend rushed to the scene and soon confirmed the unimaginable. Bodies were sprawled everywhere. Among the dead was the pastor’s 14-year-old daughter, Annabelle.