The murder gang that shot dead Detective Adrian Donohoe may have been seen beside a white HGV on a roadside in south Armagh just after the killing, gardai said.
Officers said they want to trace the driver of the lorry and anyone who was with the vehicle on the day of the shooting at the Lordship credit union in Co Louth.
Garda Donohoe was murdered in a raid which netted the gang 4,000 euro.
Detectives leading the manhunt issued appeals for information from anyone seen around the white HGV parked on Shean Road in Forkhill at about 9.45pm on Friday January 25.
Detective Donohoe was murdered minutes earlier.
Officers in Dundalk, where the extensive murder investigation is based, said the HGV may have broken down and that a number of people were seen around the lorry.
They said they also want to trace anyone involved in the recovery of the lorry or servicing it.
Gardai also want to speak to anyone who was travelling on the Shean Road in the Forkhill area at around 9.45pm on the night of Detective Donohoe's murder.
Anyone with information is being asked to contact gardai in Dundalk.
Garda Donohoe was blasted in the head with a shotgun when he confronted a gang on rural roads near his home on the Cooley peninsula. He had been accompanying cash lifts from the local credit unions.