A man is still being questioned by police investigating the murder of Bangor woman Lisa Dorrian.
Lisa Dorrian's body has never been found
The 25-year-old disappeared after a party at a caravan site in Ballyhalbert in County Down on 28 February.
Loyalist paramilitaries have been blamed for her killing. Her body has never been found despite extensive air, land, and sea searches.
An 18-year-old man was detained in the Greater Belfast area on Tuesday morning, police said.
On Sunday, her family marked her 26th birthday by releasing 26 white balloons over the seafront in Bangor.
The family said they were no closer to finding out what happened to her.
They have offered a £10,000 reward for information leading to the recovery of Lisa's body.
Three men have already been questioned about the killing but were later released.
The police are looking at the possible involvement of members of the Loyalist Volunteer Force.