Police have said they need help to catch the killers of PSNI Constable Ronan Kerr (25) who was murdered four years ago today.
No one has ever been convicted of the murder, but police said they believe with help from the local community they may be able to bring more people before the courts.
Detective Chief Inspector Una Jennings, from the PSNI's Serious Crime Branch, said that although her officers were following a number of ongoing lines of inquiry, they were seeking more information.
She emphasised that police have made strenuous efforts in their investigation over the past four years.
The senior officer said some 8,203 items have been seized; 15,539 individuals have featured in the investigation; 12,754 investigative actions have been generated; 122 searches of houses, vehicles and land have been conducted; 18 arrests have been made and one person has been jailed for offences linked to the overall investigation.
Now the PSNI want to speak to more people who may have information in the Omagh and east Tyrone area.
"From the early days of the investigation, we said this was going to be a long-term inquiry with Ronan's murder at its core but incorporating several linked incidents, including attempts to murder other police officers, a bomb attack, arms finds and armed robberies.
"It has proved to be an extremely large investigation but one which still has the potential to be concluded by charging suspects," she said.
Chief Insp Jennings said her team believe they can bring charges if they get more information.
"We have made progress and we believe there is potential to bring other individuals before the courts," she added.
"But we are realistic and we recognise the considerable challenges remaining in this lengthy and complicated investigation."
She concluded her appeal by speaking about the Kerr family's grief.
"Ronan's murder is as devastating for his family today as it was four years ago," she said.
"Their pain remains so devastating that they feel unable to talk publicly about their plight on the fourth anniversary.
"I would ask those people who know who was involved in the events and arrangements 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."
Anyone with information should contact detectives at Maydown on the non-emergency number 101 or Crimestoppers on 0800 555 111.
Constable Ronan Kerr (25) died after a booby-trap bomb exploded under his car as he left his apartment in Highfield Close at 3.45pm on Saturday, April 2, 2011. He was the second serving police officer to be killed since the Good Friday Agreement, following the murder of Constable Stephen Carroll in 2009.