Matt Vann, a six-year veteran, was hit by a hose line, knocked over and struck his head at about 3:30 p.m. while on the street responding to a fire on the 19th floor of a building at 55 Spear Street, fire officials said.
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Vann was unconscious and stopped breathing. He's listed in critical condition at San Francisco General Hospital, Fire Department Lt. Jonathan Baxter said.
Just after 10 p.m., the San Francisco Fire Department tweeted that the injured firefighter "temporarily regained altered consciousness," but remains in critical condition.
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Muni says the operator involved is a male driver with nearly 6 years experience, and they are "coordinating with SFPD on their investigation."
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Transit officials also said Muni vehicles have been re-routed around the area, and they are alerting customers to those temporary changes.
The Associated Press contributed to this story.
Stay with ABC7 News for the latest details on this developing story.