An Garda Síochána are assisting detectives from the P.S.N.I.'s Major Investigation Team with a search along the border linked to the murder of Saoirse Smyth.
Ms. Smyth was last seen in the Belfast area in April 2017.
The search operation is taking place in Omeath, Co. Louth in the Republic of Ireland.
Senior Investigating Officer, Detective Chief Inspector Darren McCartney said he believes Ms. Smyth was murdered after she returned from Belfast to Omeath where she was living at the time.
“Saoirse was originally from Belfast and disappeared in April 2017.
"Enquires have led us to believe that she has been murdered and we have been working closely with our colleagues in an Garda Síochána to get answers for her family," said D.C.I. Darren McCartney.
“Saoirse was last seen in the Belfast area on April 11th 2017 and I believe she returned to Omeath, where she was living at the time, later that day. Since her disappearance there has been no contact with family or friends which is highly unusual.
“Saoirse had very distinctive red hair and was approximately 5’0” tall and of slim build."
D.C.I. McCartney added: "Today [Monday] police are making an appeal for information about the circumstances leading to Saoirse’s disappearance and murder.
"We are particularly interested in speaking with anyone who had seen Saoirse or was in contact with her since January 2017.
“Saoirse’s family have been left devastated and they deserve to know what has happened to her.
"Anyone with any information about her disappearance should contact Detectives at the PSNI incident room Ladas Drive on 101 or 02890700355.
“Alternatively information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime," added D.C.I. Darren McCartney.