NOT former FBI! Just a former deputy sheriff, and a firearms instructor, former reserve in National Guard.
OK, but it is not solidly confirmed either way. Fox says "We are unable to confirm his prior employment with the bureau and have revised the story accordingly."
But many sources state he is a former FBI agent,
The ex-FBI agent who jumped into action when a gunman opened fire on worshippers during a church service in Texas has revealed he was placed in a position he didn’t want to be in, but had to react because “evil exists.” It comes as the shooter was named by local media as 43-year-old Keith Thomas Kinnunen. Kinnunen reportedly had a criminal past which involved arrests for theft and aggravated assault with a deadly weapon.
It was not immediately clear when he served as an FBI agent, but he said that he had taken the oath to support and defend the Constitution “multiple times” beginning in 1965, when he joined the National Guard, a reserve deputy for the Hood County Sheriff’s Department, and “multiple times” with the Department of Defence Security Clearances beginning in 1965 through 1995. “I swore to the oath then and still live by the oath today,” he wrote. https://www.news.com.au/world/north-america/at-least-two-people-dead-in-church-shooting/news-story/061d0500eccb8c0566b16533d56e1d2b