A gunman opened fire at a church in Texas on Sunday morning, killing two people with a shotgun before a member of the church’s volunteer security team fatally shot him, the authorities said.

About 250 people were inside the auditorium of the West Freeway Church of Christ in White Settlement, near Fort Worth, when the gunman began shooting just before communion, said Jack Cummings, a minister at the church.

Mr. Cummings said the gunman was “acting suspiciously” before the shooting and drew the attention of the church’s security team. The team, he said, has existed for at least 10 years and is made up of members of the church’s congregation who are licensed to carry firearms and practice shooting regularly.

“They saved a lot of lives today,” Mr. Cummings said. “Because this thing would have been a massacre otherwise.”

A member of the security team was killed in the attack, he said.

Mike Tinius, an elder at the church, said he had known that victim for more than 20 years. “He was trying to do what he needed to do to protect the rest of us,” Mr. Tinius said, adding, “It’s extremely upsetting to see anyone committing violence.”

The shooting, which the authorities said lasted six seconds, was captured on video because the church regularly posts its services online.