The investigation into the shooting dead of Detective Garda Adrian Donohoe is understood to be focused on a cross-border criminal gang, following the discovery of a burnt-out car in south Armagh.
The car, which is believed to have been used to flee the murder scene, is now being forensically examined by the PSNI.
The gang did not succeed in stealing all the money from the Lordship Credit Union in Co Louth.
As much as €40,000 in cash and cheques was left behind after the gang fled the scene with just €4,000.
The money was part of a cash collection from four credit unions in the area, which Det Garda Donohoe and his colleague were escorting to be deposited in a bank in Dundalk.
Gardaí now say that at least five men were involved in the robbery.
The gang were armed with a shotgun, handgun and hammer; they had their faces covered and were wearing tracksuits.
They blocked the exit of the credit union car park and shot Garda Donohoe, 41, in the head when he got out of his car to see what was going on.
Detective Garda Joe Ryan was held at gunpoint while the robbery took place.
Detectives are waiting to conduct detailed interviews with credit union staff and Det Ryan, who witnessed the murder.
Gardaí believe a cross-border gang suspected of carrying out a previous armed robbery on the credit union 18 months ago could have been involved in the murder.
They are liaising with the PSNI to determine whether the car is connected to the killing.
'Determined' mood among gardaí
Minister for Justice Alan Shatter said the car, found in woods at Darkley, was probably the one used by the gang.
Speaking after a visit to Dundalk Garda Station, the minister said there was a very sombre, but determined mood among members.
He said he met Det Garda Ryan and praised him for his determination to assist his colleagues to bring the killers to justice.
Mr Shatter said it is important that people engaged in criminal activity did not think it was an "open season to rob".
He described Friday's incident as a planned operation carried out by people with no respect for human life.
Speaking on RTÉ's Drivetime, the minister said it was not clear whether the group who carried out the killing and robbery "were engaged simply in organised crime" or were what he described as "criminal terrorists who fly the tricolour as a flag of convenience for their own personal financial gain".
Meanwhile, the General Secretary of the Garda Representative Association described the killing as a seminal moment in the history of the force.
PJ Stone said over 60% of the force have less than ten years of service and have never experienced the violent death of a colleague in such circumstances or walked behind the coffin of a garda shot dead in the line of duty.
Mr Stone insisted that gardaí were working in fear and trepidation and were not getting the support of the Government.
He said the force was "being decimated" and reassurances by the Government and Garda Commissioner that all resources were available to gardaí simply did not add up in this era of cutbacks.
He described the killing as a body blow and said the GRA was concerned with the immediate and long-term implications, and while now was not the time to talk about resources in the gardaí, the issue would certainly have to be addressed.
Minister Shatter said it was "unfortunate" that Mr Stone had chosen the aftermath of the death of Det Garda Donohoe to criticise the Government.
The minister said that Mr Stone should know that the murder of Mr Donohoe had nothing to do with issues of garda resources
He said: "I think any issue relating to reforms that are taking place with the Garda Síochána is an issue to be addressed on another day.
"He well knows that An Garda Síochána has the full support of the Government, the full support of myself as the Minister for Justice and they will continue to have that support."
Tributes to Adrian Donohoe
A friend of the late garda has paid tribute to him as a fine role model, who had died protecting his neighbours.
Speaking to RTÉ's Morning Edition, St Patrick's Gaelic Football Club Chairman Alan Duffy said the death was a big shock for the wider community in Dundalk.
He said as well as being a senior player, Det Donohoe had been a coach for Under 7s and Under 16s, and had been due to take up a coaching course on Saturday.
Mr Duffy said: "He's been a very visible figure in our community.
"He's been admired by everybody for years for being a fine role model, a member of Garda Síochána, a mentor to children, and a role model to teenagers.
"We are all going to miss Adrian, and it's a void that's been created in our community."
The funeral of Det Garda Donohoe will take place in Dundalk on Wednesday.
President Michael D Higgins is due to attend the funeral mass.
Mr Donohoe will be reposing at his home tomorrow afternoon before his funeral mass at St Joseph's Redemptorist Church in Dundalk on Wednesday afternoon.
Dublin Lord Mayor Naoise Ó Muirí has said a book of condolences will be opened in the Mansion House for people to sign to convey their sympathies to Det Garda Donohoe’s family.