Detectives investigating the murder of a Catholic RUC constable in Derry in January 1993 have said they are "extremely close" to a breakthrough.
21-year-old Michael Ferguson, from Omagh in Co Tyrone, was shot dead while on duty in Shipquay Street in the city at around 2pm on Satuday 23 January 1993.
He was responding to a reported theft from a local shop with two colleagues when he was killed.
Constable Ferguson had remained on the street while his colleagues entered the shopping centre to investigate the report.
He was approached from behind and shot twice in the back of the head.
The gunman made off on foot along Shipquay Street and Castle Street in the direction of the Bogside.
Detectives from the PSNI's Legacy Investigation Branch have said they are following a number of definite lines of inquiry.
The Senior Investigating Officer, Detective Inspector Conor Sweeney, said the appeal was aimed at local people who were in the area on the day.
"The gunman did not have his face covered and was wearing a distinctive green jacket, I believe that someone knows who this gunman was," he said.
He added that Constable Ferguson was "a young man murdered while responding to a reported theft from a local shop."
"I would ask everyone listening to just stop and think about that for a second. Murdered for no other reason than he was providing a service to help local businesses, local community and local people."
Detective Sweeney said it is "not too late" for people to come forward and speak to them.
Anyone with information can contact detectives in Legacy Investigation Branch on 101 or LIBEnquiries@psni.pnn.police.uk.
Information can also be provided to the independent charity Crimestoppers on 0800 555 111.