Detectives investigating the murder of Constable Michael Ferguson in 1993 say they are following a number of definite lines of enquiry.
Approaching the anniversary of the officer's death, police have said they believe they are “extremely close to progressing criminal justice action against Michael’s killers.”
The 21-year-old from Omagh was shot twice in the back of the head while on duty in Shipquay Street in the city at around 2pm on Saturday 23 January.
The officer was responding to a report of a theft at a shop in the Richmond Centre along with two other colleagues.
Constable Ferguson was keeping watch outside when he was approached from behind while the other officers were inside the building.
The lone gunman did not have his face covered and was wearing a distinctive green jacket.
The killer made off on foot entering Castle Street in the direction of the Bogside.
One man was jailed in connection with the murder.
Making a fresh appeal on the 26th anniversary of his murder detectives from Legacy Investigation Branch say they believe someone knows who the gunman was.
The Senior Investigating Officer, Detective Inspector Conor Sweeney from Legacy Investigation Branch said: “In the years since Michael’s murder, circumstances and allegiances may have changed, people’s lifestyles change and I want those who know who was involved that day to search their consciences and come forward to the police.
“I am aware that time has passed since the murder but it is not too late, if anyone now feels they are able to talk to us, we are ready to listen.
“To those involved I would appeal to them to do the right thing and make a difference to Michael’s family. They will no longer have to live with the worry that someday soon police are going to knock on their door.”
Detective Inspector Sweeney continued: “We are appealing for assistance from the local community who we still believe can help us, despite the amount of time which has elapsed. Anyone with information can contact detectives in Legacy Investigation Branch on 101 or LIBEnquiries@psni.pnn.police.uk.
"Alternatively, information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime.”