Detectives investigating the murder of Constable Ronan Kerr have made a renewed appealed for information on the 10th anniversary of his death.
Friday marks a decade since Constable Kerr's killing in Omagh on April 2 2011.
The 25-year-old PSNI officer died after a bomb exploded under his car as he left his apartment in Highfield Close to go to work in Enniskillen.
Detective Chief Inspector John Caldwell, from the major investigation team, said he still believes there are witnesses in the community who may hold vital evidence.
He said: "Ten years ago today Ronan left his home to travel to work. His job was to protect the community.
"Despicably, people living in his own community planned and plotted to kill him simply because he was a police officer bravely going out every day to protect people and make communities safer places to live and work.
"No one deserves to be murdered because of how they earn their respectable living, and I would ask those living in the Omagh area, who know vital information about the bomb attack and those involved, to bring it forward to police on this 10-year anniversary.
"It is never too late to do the right thing."
Mr Caldwell said the police officer's family deserve to have some closure after a decade of "intolerable grief".
It won't take away their pain nor will it bring Ronan back, but seeing someone brought to justice for his sickening murder will allow them to close that chapter of the process, at least
"Their suffering continues every day and they will forever miss their son and brother.
"I appeal to anyone who has information to search your conscience and bring it to police.
"I also make a direct appeal to the family and friends of the murderers to come forward. Search your conscience, as now is the time that common humanity should override misplaced loyalty."
Detectives say they have made "significant" progress in the investigation into the murder, with more than 8,200 items seized, over 15,500 individuals featuring in the inquiry, 122 searches of houses, vehicles and land conducted and 19 arrests made.
One person has been jailed for offences linked to the overall investigation.
But Mr Caldwell said officers are still 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," he added.
"However, I believe there are still potential witnesses who may be able to assist police in this line of inquiry.
"The key to putting people before a court charged with Ronan's murder lies with local communities in Omagh and east Tyrone."
Chief Constable Simon Byrne paid tribute to Constable Kerr, saying: "My thoughts today are very much with Ronan's mother Nuala, his siblings and wider family circle and friends.
"Ronan's family deserve to see his terrorist killers behind bars where they belong."
Mr Byrne said detectives have conducted a lengthy investigation incorporating several linked incidents and uncovered attempts to murder other police officers, a bomb attack, arms finds and armed robberies.
He added: "There is real potential to bring other people before the courts, but this is heavily dependent on people in the communities of Omagh and east Tyrone coming forward to talk to my officers about anything they know, particularly about the origin and sourcing of the component parts of the bomb."
Anyone with information is asked to contact detectives in the Serious Crime Branch at Maydown police station on the non-emergency number 101. Alternatively, Crimestoppers can be called on 0800 555111.