Men were seen running from a car used to shoot prison officer David Black from Cookstown dead one year ago.
The 52-year-old father-of-two was gunned down by dissident republicans on a motorway in Co Armagh as he drove to work at Maghaberry jail.
The vehicle used in the attack, a dark blue Toyota Camry, was set alight in nearby Lurgan 20 minutes later. It was in the town prior to the shooting on the evening of October 31, investigators said.
PSNI Detective Chief Inspector Richard Harkness said: “A number of men were seen running away from the Toyota Camry car.
“I would appeal to anyone who can help us with this specific inquiry or who knows anything about David’s murder to come forward.
“I would remind people there is a reward of up to £10,000 available from Crimestoppers for information leading to the arrest and conviction of those responsible.”
Mr Black, from Cookstown in Co Tyrone, was killed when the vehicle pulled alongside him at high speed on the M1 motorway and shot him close to the Lurgan exit.
The detective added: “I would also appeal to people in Cookstown to think about what they saw in the days and weeks before David’s murder last year.
“Did you see any unusual activity around David’s home or the places he frequented?
“If you have any information, please contact us.”
Serious Crime Branch detectives have made seven arrests, conducted 12 searches and completed more than 3,000 lines of inquiry.
Police conducted more house-to-house inquiries in Cookstown on Wednesday and last week a search was carried out in the Cranagh Bridge area of Lurgan.
Two men, one in Northern Ireland and one in the Republic of Ireland, face charges linked to the purchase and movement of the Toyota Camry.
Mr Harkness said: “Today is a particularly difficult day for David’s wife, Yvonne, and their children, Kyle and Kyra.
“They have endured a degree of suffering and loss over the past year which most of us cannot imagine.
“While we will continue with our inquiries, I would ask the communities in Lurgan and Cookstown to think again about what they know and provide the information which can help bring David’s killers before the courts.”