Senior cops are growing increasingly concerned that the teenage New IRA killers of journalist Lyra McKee will never face justice.
It was this that prompted the PSNI to send letters to residents of the Creggan estate in Londonderry, where the 29-year-old was shot dead a year ago yesterday, asking for their help.
In a BBC radio interview, the officer leading the murder investigation, Detective Superintendent Jason Murphy, admitted it was an extraordinary move but said he was hopeful it may yield results.
But behind the scenes, senior PSNI sources told Sunday Life the investigation was being hampered because witnesses are frightened of coming forward, even anonymously.
This fear was heightened when, in the aftermath of Lyra's killing, the New IRA put up signs around the Creggan warning "informers will be shot".
"There has been strong community support for the investigation, but no one is willing to be seen to be talking to police for obvious reasons," explained a security source.
"This has severely hampered progress. We know the name of the gunman who killed Lyra, who handed him the weapon, and who sent them onto the streets with the order to shoot at police. But proving it in court without witness statements is an entirely different matter."
A PSNI spokeswoman said that to date police have arrested nine people in connection with the murder of the journalist, who was struck by a single bullet on April 18 last year while watching rioting on Fanad Drive in the Creggan estate.
Paul McIntyre (51) has been charged with her murder on the basis of joint enterprise, with police alleging he is the masked figure seen picking up bullet casings after the fatal shot was fired. Christopher Gillen (39) has also been charged with rioting on the night of the killing.
The spokeswoman added: "Twenty-five searches have been conducted and officers are currently working through the significant amount of CCTV and other video evidence that has been recovered and conducting a thorough examination."
The gunman who shot Lyra was aged 18 at the time and has recently become a father. Despite being masked, he was recognised by locals because of the distinctive tracksuit bottoms he was wearing on the night.
He was handed the gun by another young man who is on remand in prison facing public order offences. Both were arrested by cops in the aftermath of the killing, but were freed without charge.
New IRA rioting that led to the murder was organised to impress an MTV camera crew which was in Derry filming a documentary about dissident republicans. Footage they recorded was obtained by the PSNI and forms a key part of the prosecution against Paul McIntyre and Christopher Gillen.
According to police, clothing the men were filmed wearing earlier that day is identical to that worn by two of the rioters.
The New IRA leader in Derry is Thomas Mellon. He was placed on a 10-year MI5 watch-list after being convicted of smuggling a terror note into Maghaberry Prison.
Mellon's second-in-command is veteran republican Fergal Melaugh. Both men are former Provisional IRA members.
Ultimately these are the men detectives want to charge with directing terrorism in Derry.
Detective Superintendent Jason Murphy hinted at this in his BBC Radio Ulster interview last week when he compared the murder probe to a "1,000 piece jigsaw".
There was also a nod to it in his letter delivered to Creggan residents, which read: "Lyra's murder was not committed in isolation, nor did it involve only one person. The events that led up to Lyra being shot, and the events afterwards, are equally important."
DS Murphy also confirmed 17 individuals are listed as 'suspects', and played a part in facilitating, carrying out and covering up the killing.
Lyra, from Belfast, had been living in the Creggan with her partner Sara Canning for the year previous to her murder. Ms Canning described the past 12 months as a "most horrific and surreal year" in a BBC interview to coincide with the first anniversary of her loved one's death.
She said: "I miss my life with her. If I could go back to this time last year I'd do things differently."