Investigators believe Det Garda Adrian Donohoe, who was shot while escorting a cash delivery at Lordship Credit Union, in north Louth, last Friday, may have been killed for recognizing the gunman.
Detectives are also investigating the possibility the robbers were not expecting an armed Garda escort at the delivery, reports the Irish Examiner.
Sources say the same gang is thought to have robbed the cash delivery at the same location in 2011. During that time, there was only a uniformed and unarmed police escort and the raiders got away with €62,000.
Irish police are centering their investigation on a known criminal who Det Garda Donohoe has had dealings with previously in Co Louth and who detectives suspect might be Donohoe's killer. Three other individuals, including two brothers, have also been identified as possibly being part of the gang of five.
The gang, comprised of members from their mid-teens to mid-20s, is based in the north Louth/south Armagh area.
Police are hoping that one of the five raiders will turn on the triggerman, or that somebody on the edges of the gang will leak information.
Garda sources said the group is coming under enormous pressure, not only from police, but also from serious criminals and "staunch republicans" in the area.
Police have ruled out two previously suspected brothers from North Dublin, who are believed to have murdered two innocent men in Finglas last year, and now strongly suspect the gang of five young criminals.
Garda said there was no point arresting the main suspects until they first gathered all possible information and evidence.