Police have called on the gunman who murdered an RUC officer "to search his conscience and please come forward".
Constable Michael Ferguson, from Omagh, was shot twice, at close range, in the back of the head by an IRA gunman in Shipquay Street on 23 January 1993.
The Catholic policeman was on duty at the time and was taken to Altnagevlin Hospital but died from his injuries. One man was jailed over the murder.
The PSNI have renewed an appeal for information on the "callous" murder.
"Even against the backdrop of an horrendous period in our history - I think the callousness stands out," said Det Insp Conor Sweeney, who is leading the investigation.
"He was such a young man, fresh-faced and had a look of innocence about him.
"For somebody to just walk up, not even look him in the eye, approach him from behind and snuff out that young life in the blink of an eye, it was truly horrendous.
"The family of Michael have never and I believe will never get over that."
He said the PSNI is now following a number of definite lines of inquiry, but urged anyone with information to come forward.
"I am, and remain, optimistic about a positive, successful outcome to this investigation," he said.
"But the police cannot do this alone - we do not police in a vacuum.
"We rely on and need the help of the local community. We need people to speak to us."
'We are ready to listen'
Mr Sweeney said the gunman was not wearing a mask and had a green jacket with the word "gasoline" printed on the back.
"Michael was murdered while responding to a call for help from a local shop. He was murdered for no other reason than he was providing the service to local businesses, the local community and local people," he said.
"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."
In the past, a brother of Michael Ferguson was critical of the PSNI investigation and said the family felt let down.
Police staged a reconstruction in Derry on Sunday as part of the renewed appeal for information over the killing.
It took place close to where a bomb exploded inside a car at the city's courthouse on Saturday night.