A criminal that gardai believe is a key member of a five-man gang involved in the murder of Detective Adrian Donohoe is regularly spotted by officers and is living close to where the hero officer was shot dead.
The suspect has spent most of his time living in Co Louth since the shocking 2013 murder, but gardai are powerless to arrest him because they do not have enough evidence.
"He is regularly stopped and searched but he cannot be arrested unless he is caught committing an offence. It is very frustrating but that is the reality of the situation," a source explained.
"Investigators are in no doubt that this individual was present at the scene on the night of the murder."
Sources admit there has been little fresh progress in the probe but insist the force remains "committed" to catching those responsible.
There have been no arrests directly linked to the murder yet, and officers have struggled to build a case against the suspects - who were identified within days of the shocking shooting.
Father-of-two Det Donohoe (41) was shot dead as he approached a gang involved in a robbery at Lordship Credit Union, near Dundalk, Co Louth, on January 25, 2013.
He had been accompanying cash lifts from the credit union.
He was blasted in the head with a shotgun when he confronted the gang on rural roads near his home on the Cooley peninsula.
Investigating officers identified the gang responsible within days.
Gardai have been assisted by PSNI officers because much of the gang were based in the North.
Those involved in the murder are from Crossmaglen, Co Armagh, and its surrounding areas on the border with Co Louth.
The suspected shooter is understood to be now based in Boston after being rejected by other criminals when he relocated to New York after the shocking crime.
His girlfriend has provided him with an alibi for the night that Detective Donohoe was murdered.
It is understood that the Criminal Assets Bureau have investigated the gang involved in the crime but were unable to identify any major assets linked to the mob.
Garda Commissioner Noirin O' Sullivan spoke on the second anniversary of the murder earlier this year.
"This is a murder investigation and, as with any murder investigations, it is very complex. It is very complicated," she insisted.