Kavan Brissett, who would have turned 22 last week, was knifed in his chest in an alleyway off Langsett Walk on August 14 and died in hospital four days later.
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Detectives investigating the stabbing believe Ahmed Farah, 29, who has links to Broomhall, could hold vital information about the killing and have circulated him as ‘wanted’.
They have released his name and photograph and a £5,000 Crimestoppers reward for information is available for information leading to his arrest.
South Yorkshire Police claim Farrah knows he is wanted for questioning and is deliberately evading arrest.
In a Facebook post, Kavan’s aunt, Libby Hamilton, claims Farrah has not been reported missing by his family despite him disappearing six months ago.
She said: “If you hadn’t seen or heard from your son or friend for nearly six months, would you file a missing persons report? I would.
“Ahmed Farrah’s family have not. There is no report of him missing despite no one knowing where he is or hearing from him since 14.08.18... strange.
“If you’ve done nothing, hand him/ yourself in and let the investigations move forward.
“A lot of police effort is going into locating someone that, if he’s as innocent as family say, could be spent on others. Name could be easily cleared, if that’s the case.”
She added: “Do Farrah's family think there's a risk Farrah could be dead or in grave danger?
“Six months and ostensibly no contact from him. Police can't even find him, and nobody's heard a peep.
“Whether or not a relative of mine had committed a crime of this nature, I'd still file a missing person's report because of the possibility that my relative was innocent and because I cared about the safety of my relative since there is a thing as innocent until proven guilty.”
Anyone with information should call South Yorkshire Police on 101 or Crimestoppers, anonymously, on 0800 5551.