Authorities said more than 240 parishioners were in the church at the time of the shooting.

White Settlement Police Department Chief JP Bevering said the gunman had sat down in a pew before getting up, taking out a shotgun and firing at a parishioner, who was killed. He said the church's security team then "eliminated the threat".

Officials had not released the names of the victims or the gunman, as of Monday afternoon Australian time. FBI Special Agent in Charge Matthew DeSarno said the FBI was working to identify the gunman's motive, adding that he was "relatively transient" but had roots in the area.

DeSarno also said the gunman had been arrested multiple times in the past but declined to give details.

In a livestream of the church service, the gunman can be seen getting up from a pew and talking to someone at the back of the church before pulling out a weapon and opening fire. Parishioners can then be heard screaming and seen ducking under pews or running as papers fly to the floor.

Two people with minor injuries sustained while ducking for cover were treated at the scene, MedStar Mobile Healthcare spokeswoman Macara Trusty said.

"Unfortunately, this country has seen so many of these that we've actually gotten used to it at this point. And it's tragic and it's a terrible situation, especially during the holiday season," Jeoff Williams, a regional director with the Texas Department of Public Safety, said at a news conference.

"I would like to point out that we have a couple of heroic parishioners who stopped short of just anything that you can even imagine, saved countless lives, and our hearts are going out to them and their families as well."

An elder at the church told The New York Times that one of those killed was a security guard who responded to the gunman, calling the guard a dear friend.

"He was trying to do what he needed to do to protect the rest of us," the elder, Mike Tinius, said.

"It's extremely upsetting to see anyone committing violence," he said.

Tinius said he didn't know the gunman and that the shooting appeared to be random.

A woman who answered the phone at the West Freeway Church of Christ said she could not answer any questions and that she was told to direct inquiries to authorities.

Texas Governor Greg Abbott asked the state to pray for the victims, their loved ones and the community of White Settlement.

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"Places of worship are meant to be sacred, and I am grateful for the church members who acted quickly to take down the shooter and help prevent further loss of life," Abbott said in a tweeted statement.

In 2017, Texas passed a law allowing churches to hire armed guards, the Fort Worth Star-Telegram reported. Another law that went into effect earlier this year lets licensed handgun owners bring their firearms into church as long as it's approved by the church.

It is not the first deadly shooting to take place at a church in Texas. In November 2017, Devin Patrick Kelley opened fire on the congregation at a church in Sutherland Springs, Texas, killing more than two dozen worshippers, before taking his own life.

And in 1999, a gunman killed seven people in Wedgwood Baptist Church in Fort Worth before detonating an explosive device and killing himself.