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.
In the video, the gunman stands up during a quiet moment and briefly talks with someone standing against a wall.
special agent in charge of Dallas described as a transient person with roots in the area.
The agent, Matthew J. DeSarno, said the investigation into a motive was continuing.
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