Police have made a new appeal for information about the disappearance of a Co Down woman 15 years ago.
Lisa Dorrian was last seen alive at Ballyhalbert Caravan Park on February 27, 2005.
Detective Superintendent Jason Murphy said he was making a fresh plea for information in a bid to help relieve the Dorrian family's anguish and "allow them to lay Lisa to rest".
Ms Dorrian was 25 when she disappeared and police believe she was murdered.
Detective Superintendent Jason Murphy said: "Somebody knows what happened and it is not too late for those people to come and talk to me and my team.
"I know that individuals will have wrestled with their conscience for many years and today I am giving an opportunity for them to put that conscience to rest too.
"My direct appeal is to those within that close circle of knowledge to contact us."
Despite extensive searches and numerous appeals for information, Ms Dorrian has not been found.
I want the killer to know that we have pursued them for 15 years and we remain resolute in seeking justice for Lisa's familyDetective Superintendent Jason Murphy
Mr Murphy said no one had been brought to justice in connection with her disappearance. The detective added: "Every anniversary for the Dorrian family is incredibly difficult, but with each year that passes their determination to find Lisa increases.
"It is now 15 years since Lisa inexplicably vanished and our thoughts today are with her family and friends.
"She was last seen alive in the caravan around 10pm on Sunday February 27.
"I believe she was murdered that night or in the early hours of the following morning."
The PSNI believes Ms Dorrian knew her killer.
Mr Murphy said: "There is nothing to suggest that Lisa came to harm at the hands of a stranger. I have always believed that a small number of people hold the key to us finding out what happened to Lisa.
"Those individuals must live forever with a heavy conscience because at this time every year the Dorrian family will renew their vigorous and unending search for Lisa.
"The police service stands right alongside them and our determination remains equally resolute.
"This has been a significant investigation to date and it remains absolutely active.
"We have conducted searches recently and will continue to identify other areas in which Lisa or clues might be.
"I want the killer to know that we have pursued them for 15 years and we remain resolute in seeking justice for Lisa's family."
Anyone with information can call detectives on 101 or Crimestoppers on 0800 555 111.
Police have pursued more than 3,500 lines of inquiry and carried out almost 400 searches, including extensive air, land and sea operations, since Ms Dorrian's disappearance.
Last October it was reported that PSNI detectives now have just one suspect in mind - a man who they believe killed Lisa and then summoned his father to help dispose of the shop assistant's body.