Detectives hunting the killers of German backpacker, Inga Maria Hauser, have received over 30 calls from members of the public as a result of their appeal last month and have carried out further DNA testing.
The body of the teenager was found in Ballypatrick Forest, outside Ballycastle, Co Antrim, 15 days after she boarded the ferry from Scotland.
Last month, police launched a new appeal to coincide with the 30th anniversary of Inga Maria's death while relatives of the young woman have also appealed for help.
An inscribed memorial stone was also erected at the spot where her body was found.
Detectives believe a number of people may have been involved either directly or in the cover-up, and have previously said they only need fractional pieces of evidence to bring the chief suspect or suspects to justice.
A man in the rural area east of Ballymoney in Co Antrim was seen soon after the murder with scratches on his face, sparking rumours in the community that he had been involved.
The officer in charge of the investigation, Detective Chief Superintendent Raymond Murray, said: “Last month on the 30th anniversary of Inga Maria's murder, police launched an operation spanning Northern Ireland and Scotland during which we followed a number of specific lines of enquiries.
"We appealed for information as we believe members of the public may hold vital information about who killed the teenager and as a result received over 30 calls which we are currently assessing.
“Some of the developments over this period have been interesting and are being progressed by the investigation team.
“One strand of the operation was to identify the man whose DNA was found at the crime scene. A previous DNA screening process was one of the largest ever conducted and involved many hundreds of people, throughout the UK.
"However, as time evolves so does DNA science so we have carried out further familial DNA screening, [which uses genetic material from relatives to track down a suspect]. We have now received the results and have met with scientists and other specialists in order to take these forward.
“We still believe we are extremely close to progressing criminal justice action against Inga Maria’s killer or killers.”
It has been claimed that the IRA investigated the murder at the height of the Troubles and considered passing on information to police.
East Derry MLA John Dallat said he believed there are people who know what happened to Ms Hauser.
He urged them to "do the right thing" and talk to police. "This wee girl was brutally beaten to death and the cover-up is unforgiveable," he said.
The Press Association reported a source who said: "They (the IRA unit) had identified somebody that was responsible from the Loughgiel area and we believe they had a debate among themselves about whether that information should be passed in one way or another to the police."
But the source added: "There may well be other people who were privileged to the debate that was held.
"We are now 30 years down the line. A lot of things have changed. A lot of water has gone under the bridge.
"These people may still have that knowledge. That debate indicates that they wanted this resolved.
"The fact that they were debating it in 1988 highlights that they thought this was something different."
Detective Chief Superintendent Murray said Ms Hauser's deserve closure after 30 years.
“Her father died not knowing who killed his daughter and her mother has been ill for many years. The family have been tortured by her murder and we have been in close contact with Inga’s heartbroken sister in the run up to and following this anniversary.
Friederike Leibl asked the PSNI to issue this letter on her behalf:
"It's been nearly 30 years since my younger sister Inga-Maria was so brutally murdered, you cannot imagine how the murder of my sister has affected my family - we are broken inside. My father was worst hit - he fell ill with prostate cancer in 2005 and was treated for depression when the whole body was full of cancer.
"The fact of being confronted with this murder for a lifetime is just cruel, sick, sick. So I ask you to come forward within formation, otherwise my sister's soul will never rest."
Detective Chief Superintendent Murray added: “I also continue to make a direct appeal to the family and friends of the murderer or murderers to come forward. Thirty years has passed and in that time we have to ask not only what impact this murderer has had on the Hausers but what impact has he had on his own family. How has it affected them? If you are a family member who has information or who even assisted the killer or killers in the aftermath of the incident, search your conscience as now is the time that common humanity should override misplaced loyalty.
“Give us the information we need to take this investigation forward and place him before the courts. What if this was your daughter or granddaughter – subjected to a brutal and ruthless assault after arriving in a new country before being killed and left in a forest. Think of the fear and pain she felt, think of her family not having justice.
“After 30 years, it's time to tell us what you know - contact Police by calling detectives in Maydown on 02871379783.”
