The specimens were found in 2012 at a derelict flat in Main Street, Castlederg, Co Tyrone.Announcing the discovery on Thursday, the 20th anniversary of Arlene's murder, Detective Superintendent Karen Baxter said: "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."Arlene Arkinson, who was 15 years old, was last seen in the early hours of 14 August 1994. She disappeared after a night out in Bundoran, Co Donegal and her body was never found.Convicted child killer Robert Howard, who was the last person seen with her, stood trial in 2005 accused of her murder, but was found not guilty.Police said tests have confirmed that the blood specimens discovered two years ago do not belong to the missing girl, but are from two different women.Neither of them are registered on the national DNA database.D/Superintendent Baxter continued: "They have a story to tell and we want to hear it. Their assistance could be beneficial to the investigation."Police added that they will be carrying out a number of land searches across Tyrone and other areas in the coming months as part of efforts to find Arlene's remains and further the investigation.A man has also been interviewed in England.D/Superintendent Baxter said: "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."Information can be passed to the PSNI by calling the special incident room on 028 7137 9783.