Detectives investigating the murder of Arlene Arkinson have found blood traces from two other females.
A derelict flat at Main Street in Castlederg was searched in 2012 after new information became available to investigators.
Specimens of blood belonging to two mystery women were found during the search.
The officer now leading the investigation, Detective Superintendent Karen Baxter, disclosed the development today - on the 20th anniversary of the Tyrone teenager’s murder.
Arlene was last seen in the early hours of 14 August 1994. The 15-year-old disappeared after a night out in Bundoran. A 61-year-old man stood trial for Arlene’s murder in 2005 but was acquitted.
Det Superintendent Baxter said: "These blood specimens belong to two different women. Tests have confirmed they are not from Arlene.
"But neither specimen has registered on the national DNA database.
"On the 20th anniversary of Arlene’s disappearance, I would ask that any woman who was in this flat, at 49a Main Street which has since been demolished, at any time to make contact with police. They have a story to tell and we want to hear it. Their assistance could be beneficial to the investigation."
Officers will be conducting a number of land searches in various parts of Tyrone and other locations over the coming months in an attempt to recover Arlene’s remains and progress the investigation.
Enquiries have taken detectives to England to interview a man, said DS Baxter.
She said: “This investigation will continue. It will be progressed as far as we can take it. Indeed, I can confirm that our enquiries have involved detectives interviewing a man in England.
“While we acknowledge the pain this causes to Arlene’s family, police alone cannot solve this case or find Arlene. It is only with the help of the public that the person responsible will be brought to justice and the Arkinson family will find any degree of comfort.
“Arlene’s family is being kept informed of these developments. We understand as much as we can their continuing grief and frustration. We will continue to do our utmost to bring this to a conclusion within an extremely complex investigative environment and diminishing opportunities. We would ask anyone with information to come forward.”
Searches will be carried out by a small team assisted by dog handlers and body recovery dogs.
The special incident room number to call is 028 7137 9783.