Lisa Dorrian, was last seen alive on the night of February 27, 2005, at Ballyhalbert Caravan Park in Co Down.
Police believe she was murdered that night or in the early hours of the following morning.
Detective Chief Inspector Kerrie Foreman, from the PSNI’s Major Investigation Team, said: “Lisa, who was from Bangor, was last seen alive on the night of Sunday February 27 2005 at a party in Ballyhalbert Caravan Park.
It’s believed she was murdered that night or in the early hours of the following morning.
“Despite hundreds of extensive search operations and thousands of lines of enquiries completed, Lisa’s body has not been found, and those responsible for her murder have not yet been held to account.
“Twenty years have now passed. That’s 20 long years of unimaginable torment for a loving family. Yet, despite their pain, they’ve never given up their search for answers and, likewise, our determination has remained steadfast.
“We’re releasing CCTV footage, along with still images.
“This is the last known footage of Lisa in the run-up to her disappearance. My hope is to jog memories, and to encourage anyone with information to speak up.”
You can contact Crimstoppers anonymously on 0800 555111