Detectives investigating the murder of Constable Ronan Kerr believe tracing bomb parts may hold the key to putting his killers behind bars.
The PSNI officer, 25, was killed by a booby-trap car bomb in Omagh, Co Tyrone, on April 2 2011.
Despite the largest probe in the force’s history, charges are yet to be brought over the murder, and investigators are appealing to the community to provide what they hope could be the vital clue.
Detective Chief Inspector John Caldwell said: “Detectives have made a significant amount of progress in the investigation into the murder of Ronan, however we are particularly interested in the origin of some of the component parts of the bomb, and how they were sourced.
“We have already made good progress in tracking how the terrorists came into possession of these parts, as well as establishing their relationship with a local crime gang in the Omagh area.
“However, I believe there are still potential witnesses who may be able to assist police in this line of enquiry.
“The key to putting people before a court charged with Ronan’s murder lies with local communities in Omagh and east Tyrone.”
Police said more than 15,500 people have featured in the investigation, with more than 8,200 items seized and 122 searches conducted of houses, vehicles and land.
Some 19 arrests have been made and one person has been jailed for offences connected to the probe.
Mr Caldwell added: “Ronan’s family deserve to see his terrorist murderers behind bars where they belong. They remain as devastated as they were when he was killed nearly seven years ago.
“This has been a lengthy and complicated investigation incorporating several linked incidents including attempts to murder other police officers, a bomb attack, arms finds and armed robberies.
“There is significant potential to bring other individuals before the courts and we need people in the communities of Omagh and east Tyrone to come forward with anything they know, particularly about the origin and sourcing of the component parts of the bomb.
“Although we have yet to bring charges for Ronan’s murder, this investigation, which is the largest in the PSNI’s history, is far from over.
“We will continue to pursue our live lines of enquiry.
“I would urge those people who know who was involved in the events and activities surrounding Ronan’s murder to come forward.
“If you can assist in any way by providing information, please do so. It is the right thing to do. Ronan and his family deserve justice.”