Co Louth and surrounding counties.
Armed patrols surrounded the immediate area and extra officers were drafted into Dundalk to join the manhunt.
The murdered detective's partner is not understood to have been injured in the incident.
It is understood that four men in a dark-coloured car were involved in the attempted robbery. A Passat car left the scene towards Dublin at high speed.
The unmarked garda car was riddled with bullets during the incident.
The spot where the robbery took place is a short distance from the Border.
A huge security operation immediately swung into place as specialist units raced from Dublin and the PSNI attempted to seal off the Border.
Heavily armed Emergency Response Unit members rushed to the scene while the garda air wing was also alerted.
Gardai set up roadblocks in an attempt to trap the killers.
Last night Ann McCabe, widow of murdered Garda Jerry McCabe, said she wanted to pass her sympathies on to the family of the slain garda.
"It is 17 years later on and history is repeating itself. We all thought those dark days were left behind," she said.
"I have to sympathise with that family and the shock they are going through tonight."
The detective is the first garda shot dead since Gda McCabe was killed as he and his colleague, Detective Ben O'Sullivan, were escorting a post office van in 1996.
They were parked in a garda car behind a van in Adare when a jeep crashed into them from behind.
In a murder that shocked the nation, two members of the Provisional IRA got out of the vehicle and fired on the garda car with an AK-47 assault rifle.
Gda McCabe was hit three times and his partner was shot 11 times but miraculously survived his serious injuries.
Four men were convicted in connection with the crime but charges were reduced to manslaughter after two witnesses withdrew their testimony.
Gda McCabe's name, along with those of 82 other gardai who died in the service of the State, is recorded on the Garda Roll of Honour.