Fahim Hersi was knifed in an attack outside Cineworld on Friday, September 21, last year, but nobody has yet been charged.
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Fahim, who was stabbed in his chest, was rushed to hospital but could not be saved.
A number of arrests have been made over the stabbing, with a 21-year-old man who was also injured during the incident among those quizzed over the death.
Temporary Detective Chief Inspector Simon Palmer said: “We still have a full team investigating the murder and we are pursuing every active line of enquiry to ensure the perpetrators of the offence are brought to justice.
“Fahim’s family is devastated at their loss and want a conclusion to the investigation as quickly as possible.”
He added: “This happened at a busy entertainment complex and as such there were a lot of people around, some of whom have not yet come forward.
“It may be that they don’t realise how important the information they hold is.”
He said officers are still analysing CCTV footage from Centertainment and across the city as they piece together the circumstances surrounding the attack.
DCI Hersi said: “It is critical that we find those involved. This was a knife crime in a public place and public safety is our priority.”
Anyone with information should call South Yorkshire Police on 101 or call Crimestoppers, anonymously, on 0800 555111.
Quote incident number 950 of September 21.