A gang of hardened Dublin criminal based in Newry and Warrenpoint, Co Down, were the first port of call for officers investigating the murder of Det Gda Adrian Donohoe.
PSNI officers from Newry visited the homes of a number of the gang members within an hour of the cold-blooded killing on Friday night.
Police officers received no response at the addresses they visited and an alert has been issued to locate the men.
The criminals would have been well known to Det Gda Donohoe, who had been actively involved investigating serious crime along the Border.
The gang are believed to have robbed the credit union in 2011 when they got away with €62,000 in cash. Then, the raiders were armed with sawn-off shotguns and their getaway car was also abandoned just north of the Border.
This is centred on two notoriously violent brothers who are originally from Coolock.
However, investigators must check their suspected involvement against the fact the killers apparently spoke with "local accents".
The gang have been involved in murder, raids and drugs.
It includes up to 16 associates who are mostly from Kilbarrack and Coolock in north Dublin but now living in the Newry area.
The leaders moved their operation to Co Down over three years ago following a bloody feud with former associates in Dublin, which has so far claimed seven lives.
The gang, which has been on the gardai's most-wanted list for over three years, is ranked as one of the most dangerous in the country.
The criminals had been involved in a bloody feud, which began with the theft of €1m from a security van in Bettystown, Co Meath, in July 2004.
When two members of the gang "stole" €300,000 from another gang member, it caused a bitter rift that descended into shootings and then murders.
The Coolock brothers and their group are suspected of murdering three of their former associates and the attempted murders of two others.
The same gang are also the chief suspects for the double murders of innocent cousins Glen Murphy (19) and Mark Noonan (23) who were gunned down at the Tesco Clearwater filling station in Finglas in November 2010.
The two men, who were mistaken for two of the feuding criminals, were clinically executed by two gunmen.
Detectives investigating the double murders have been working closely with the PSNI in recent years and they are hopeful that the brothers and a number of other associates will be charged with the crimes.
The same gang also featured in another double gangland murder near Dundalk in March of last year. Small-time Dublin drug dealers Joe Redmond (25) and Anthony Burnett (31) were lured to meet members of the gang at Ravensdale.
The two men had been instructed to steal a car in Sandymount, south Dublin, supposedly for use in an armed robbery. Redmond and Burnett were both shot before their bodies and the stolen car were incinerated by the killers.
Meanwhile, Adrian's colleagues have been getting on with the difficult job of investigating his murder.
A local source said: "It is mind-numbing and heartbreaking but the best thing we can do for Adrian is to be as professional as he was and catch these people. He would do the same for any of us."
The senior officer in overall charge of the investigation, Det Supt John O'Reilly, was a neighbour of Adrian's growing up.
"They practically grew up together and you can imagine what that means for John. But he is an absolute professional and will get the job done," said a colleague.