Lisa Dorrian, a shop assistant from Bangor, County Down, disappeared after a party at a caravan site in 2005. Widely believed to have been murdered, her body has never been found.
BBC News NI looks at the search for Ms Dorrian.
28 February 2005: Lisa Dorrian goes missing
The 25-year-old disappears after attending a house party at a caravan park in Ballyhalbert, County Down. She is last seen at 05:00 GMT.
Her handbag and belongings are discovered at the party.
6 March 2005: Family's first appeal
Ms Dorrian's father, John, appeals for information about the disappearance of his daughter six days after she goes missing.
"If she is out there and sees all this involvement with the police and the media, it doesn't matter. Don't worry about it, come back."
Police say they cannot rule out that a crime has taken place.
7 March 2005: Search stepped up
The search for Ms Dorrian intensifies a week after her disappearance, with extensive air, land and sea searches along the Ards Peninsula.
Police carry out a reconstruction at 05:00 GMT, timed to the exact hour Ms Dorrian is believed to have gone missing.
An ex-boyfriend of Ms Dorrian's, Jimmy Mills, said she had been spending more and more time in the Ballyhalbert area.
"She had started to run about with different people and she started hanging about down round this area," he said.
"I used to be with her all the time, day in and day out. Now she has moved on in her life. She was my first love and I just hope she's all right."
13 March 2005: Murder investigation begins
There are also fears that alleged loyalist paramilitary involvement is preventing people coming forward with information.
Graffiti appears in Ballyhalbert linking the murder to the paramilitary Loyalist Volunteer Force (LVF).
28 April 2005: £10,000 reward offered
Ms Dorrian's family offer a £10,000 reward for information leading to the recovery of her body.
John, her father, said her death had "ripped the family apart" and that they could not move on until they "give her a Christian burial".
17 May 2005: 'No doubt LVF killed Lisa'
A BBC Spotlight investigation reveals loyalist paramilitary groups are carrying out a parallel investigation into the death of Lisa Dorrian.
The programme reports that members of the Ulster Volunteer Force (UVF) and Red Hand Commando had interrogated two teenagers over her disappearance, and believed the LVF are behind the crime.
David Ervine, then leader of the Progressive Unionist Party which is closely linked to the UVF, said he had no doubt that Ms Dorrian was killed by the LVF.
"I mean, the smart money says that it's a very tiny number of people and, if you push them hard enough, people will name them to you. So names are being bandied about. Let's not kid ourselves."
Ms Dorrian's mother, Pat, said their family is willing to speak to loyalist paramilitaries about finding Lisa but "don't want any retaliation and no comeback".
27 May 2005: Conflicting accounts over phone call
The BBC learns that two men questioned over the murder of Ms Dorrian gave different accounts of a phone call said to be made moments after she disappeared.
A 19-year-old, who claimed to be the last to see Lisa, said the pair were frightened by noises outside the caravan where they were partying, and that they ran away and he lost Ms Dorrian in the dark.
He then called her mobile phone and spoke to her boyfriend who, he said, told him he was in Bangor, County Down, with another woman.
However, the man who answered Ms Dorrian's phone told police he was with another man and teenage girl in a flat in Ballywalter, and that Ms Dorrian's phone had been left in the flat.
Meanwhile, the BBC reports that two separate allegations were made regarding Ms Dorrian's death during two meetings at Belfast City Hall - that she was being harassed by two men over money for drugs in the weeks leading up to her death and that she was minding £20,000 at her Bangor flat for another man.
It's reported that details of these allegations were given to David Ervine who passed them to police.
31 May 2005: Family meet with David Ervine
The family said that Mr Ervine stressed his involvement was in order to help the family and that he had no political agenda.
Mr Ervine described the circumstances around the murder as a cesspit and added: "If you leave cesspits alone, they multiply."
12 June 2005: Balloons released for birthday
Ms Dorrian's family mark what would have been her 26th birthday by releasing 26 balloons over Bangor's seafront.
1 July 2005: Fundraising appeal continues
Blue ribbons are given out to support attempts to find her body.
To date, four people have been questioned about the murder but all were freed without charge.
16 September 2005: Police make boat appeal
Police say they believe Ms Dorrian's body may have been hidden in water and appeal to boat owners in the Ards Peninsula to check their vessels for signs they had been tampered with or broken into.
23 August 2006: Snow Patrol support appeal
Rock band Snow Patrol pledges its support to the efforts to find Ms Dorrian's body by agreeing to wear blue ribbons during their performance at major rock concert in Belfast, after Lisa's sister, Joanne, contacts them online.
11 December 2007: 1,000 days since Lisa's disappearance
More than 1,000 days since Ms Dorrian's disappearance, her father, John, says the family still felt "just like it happened yesterday".
As the family prepared for another Christmas without her, he said they still hoped that someone would come forward with information.
"We would appeal to them in humanity, please any small bit of information, give it in confidence.
"It is a gnawing in your stomach, you know she is out there somewhere. It would give us great relief if we could just know where she was."
16 October 2012: Police search for vehicle
Police investigate an area of farmland near Comber, County Down, as part of a search for a vehicle possibly used in the murder of Ms Dorrian.
23 February 2015: New reward offered on 10th anniversary
Shortly before the 10th anniversary of Lisa Dorrian's disappearance, her family once again appeal for information.
Meanwhile, Crimestoppers offers a new reward of up to £5,000 for information leading to the arrest and conviction of those responsible.
This new reward is in addition to the £10,000 originally offered by her family.
28 June 2015: Convicted murderer makes body claim
A prisoner serving a life sentence for murder claims to a Sunday newspaper that he knows where the body of Lisa Dorrian is buried.
Jimmy Seales told the Sunday Life she was buried in a sealed container on an illegal landfill site near Ballygowan, County Down.
5 January 2016: Funeral of Pat Dorrian, Lisa's mother
The funeral of Pat Dorrian, Lisa's mother, takes place following her death in late December.
Mourners are told that the death of her daughter "took an unbearable toll" and that while her wish to see Lisa buried would not be fulfilled for her, "it is one that can still be fulfilled for her family".
15 February 2016: Comber farmland searched for body
The search ends a week later with no human remains being found.
28 February 2017: Fresh appeal on 12th anniversary
Ms Dorrian's father and sisters renew their appeal on the 12th anniversary of her disappearance, and record a video message addressed to the "person who was with Lisa at the moment she died".
"It's never too late to tell us. A place is all we need," said John Dorrian.
"Please help yourself by easing your conscience. Tell us where Lisa is."
29 June 2018: New sites searched after fresh appeal
Following an appeal on BBC's Crimewatch fresh police searches are carried out.
Appearing on the programme, Lisa's sister Joanne Dorrian said the family had endured "13 years of torture".
"We lost my mum about two and a half years ago and she just couldn't cope with not having Lisa," she said.
Police search three new sites, including wooded areas in Craigantlet and Carrickfergus using specially-trained police dogs.
01 April 2019: Fresh searches at disappearance site
Police announce fresh searches of the caravan park in Ballyhalbert, County Down, where Ms Dorrian was last seen alive.
Officers also search a disused airfield behind the caravan park.
Announcing the searches, head of the investigation Det Supt Jason Murphy said: "The determination of the PSNI to bring those who killed Lisa Dorrian to justice is as strong today as it has ever been."