A man with Tourette's syndrome was hit repeatedly around the head with a hammer in a "cowardly" attack police believe may be linked to his condition.
Jonathan Mars, 27, suffered a fractured skull and nose and severe bruising after he was set upon as he got off a bus in Lowedges Road, Sheffield.
South Yorkshire Police said it was investigating links between the assault and Mr Mars' Tourette's.
Mr Mars said he was left feeling "terrified" after last month's attack.
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Supt Colin McFarlane said: "Jonathan is a vulnerable person. He has suffered a nasty attack and one of the things we are pursuing is that it was a result of his Tourette's.
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"I think they are a coward in what they have done and I'm determined that my officers will do all they can to ensure he gets arrested."
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