Tens of thousands of hours of CCTV footage have been seized by detectives investigating the murder of policeman Ronan Kerr in Northern Ireland.
Investigators appealed for anybody with video footage of part of the Omagh half-marathon, just before an under-car booby-trap bomb killed Catholic officer Ronan Kerr on April 2, to give them the material.
The device was planted yards from the route in the Co Tyrone town by dissident republicans opposed to the peace process.
Investigating officer Detective Superintendent Raymond Murray said it was going to be a long and protracted investigation.
"But we have been encouraged by the co-operation which we have received from virtually everyone with whom we have engaged and heartened by their willingness to provide whatever information they have," he added.
Constable Kerr, 25, was a new recruit to the force.
Four people have been arrested by police and one man charged with possession of items likely to be of use to terrorists and possession of firearms and explosives with intent.
More than 30 searches have been carried out in Northern Ireland, the Republic of Ireland and Scotland.
Mr Murray said tens of thousands of hours of CCTV material have been seized.
"Anyone who thinks they may have relevant photographic material from the half-marathon taken between 1.15-1.45pm in the Highfield area is asked to contact police at Omagh," he said.
More than 4,000 items have been seized during searches and are now being examined in forensic laboratories.
Detectives have visited 780 houses and completed 1,160 questionnaires of people living in them.