It has emerged that the gang that shot Detective Garda Adrian Donohoe dead then held his colleague, Detective Garda Joe Ryan, at gunpoint while they robbed the Lordship Credit Union in Dundalk, Co Louth.
The gang stole €4,000 in cash before escaping from the scene.
It is believed that the raiders knew the credit union money was being escorted by gardaí.
While gardaí have not ruled anything out, they suspect that a cross-border gang was involved in the robbery and murder, and that the gang operates in the north Louth-south Armagh area.
A burnt-out car, possibly a Volkswagen Passat, was found close to Keady in south Armagh and is being examined by forensic teams in Northern Ireland.
The same gang is suspected of having stolen €62,000 from the same credit union a year-and-a-half ago, also using shotguns and on a Friday before the garda escort arrived.
Over 100 gardaí are now involved in the investigation, with officers cancelling leave and coming in on their days off. Gardaí from all over the country have also offered to come to the town to help.
Earlier, Garda Commissioner Martin Callinan stressed that now is the time for those with information to come forward because other people know the men involved.
Meanwhile, the post mortem on the body of Det Garda Donohoe has been completed at Our Lady of Lourdes Hospital in Drogheda.
His funeral is due to take place on Wednesday.
Prayers were said at masses in Lordship and across the country today in his memory.
In his home village of Kilnaleck, where he played Gaelic football as a boy and at minor and under-21 level for Cavan, his murder has caused immense shock and grief.
Politicians condemn killing
Minister for Communications Pat Rabbitte has condemned the killing of Det Garda Donohoe, calling it an "appalling cowardly murder".
Speaking on RTÉ Radio, the minister said the shooting did not happen because of the closure of garda stations or cuts in the Budget.
Mr Rabbitte said the shooting was a "cold-blooded assassination" that demonstrated how gardaí put their lives at risk every day.
He said the criminal underworld in Ireland has reached "a new low" when a garda with no weapon tries to make an inquiry and the response is to shoot him dead.
Sinn Féin leader and Louth TD Gerry Adams has said that anybody who has information relating to the shooting of Det Garda Donohoe should bring it to gardaí.
Speaking on RTÉ's This Week, Mr Adams said if there was a Northern Irish element to the killing, then anyone with any information should go to the PSNI.
Mr Adams extended his condolences to Mr Donohoe's colleagues, wife, family and friends.