PSNI detectives have made a renewed appeal for information about the murder of Constable Ronan Kerr.
Constable Kerr, 25, died in a booby-trap car bomb in Omagh on 2 April 2011.
The attack was carried out by dissident republicans.
The officer leading the investigation said that although detectives were following a number of ongoing lines of inquiry he believed that "there are witnesses in the community who may hold the key to vital evidence".
Det Ch Insp 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," he added.
Police believe the attack may have been linked to other attempts to murder police officers, as well as a bomb attack, arms finds and armed robberies.
They said 122 searches of houses, vehicles and land have been conducted and 19 arrests made.
One person has been jailed for offences connected to the investigation, however, no-one has been charged over Const Kerr's murder.
"Ronan's family deserve to see his terrorist murderers behind bars where they belong," Det Ch Insp Caldwell said.
"They remain as devastated as they were when he was killed nearly seven years ago.
"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."