Shocking footage shows a teenager casually setting a woman’s hair on fire while she sits with her back to him on a bus in San Francisco.

The teenager – all dressed in red – sauntered over to sit next to the woman who was looking ahead, apparently unaware of what was happening.

The footage released by the San Francisco Police Department shows the cruel teenager holding a lighter to the back of the woman’s hair for a few seconds – at some point even taking it away to see if it was working properly.

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The stranger’s hair was then “set on fire” and was helped by other passengers when the fire bug and two buddies fled with him, police said.

It wasn’t immediately clear how badly she was injured, as the victim – described as a Filipino or Hispanic woman in her 50s or 60s – also left before police on Jan.

The police appeal not only sparked outrage, it stunned many spectators – because the twisted fire starter’s face was blurred, making it nearly impossible for witnesses to identify.

“You want us to help you but blur the thug’s face!” one person complained to the police on Twitter. “The attacker would probably have been caught if his face had been shown first!”

Police spokesman Adam Lobsinger later told SFGate that they would first focus on finding the victim so they could get testimony to help set up a case.

Without the victim, it is more difficult to do anything if we catch the suspects, “said Lobsinger.

“We’re still looking for the suspects,” he said, describing the captain as a black teenager in a red outfit and a black backpack.