Two detectives from Ireland have flown to the United States in an effort to track down the chief suspect in the murder case of Garda Detective Adrian Donohoe.

Donohoe, 41, was shot and killed during a botched robbery by a gang of thugs near the Northern Irish border, in County Louth on January 25. Irish police have been on the hunt for the cop killer suspect, a man in his 20s from South Armagh, since he fled Ireland in April.

The suspect, believed to have been the trigger man, fled to the United States and officers have been tracking him over the last few weeks in New York, according to the Irish Mirror.

"Two detectives from the investigation team were sent over to New York a few weeks ago to keep a close eye on the suspected trigger man in Garda Donohoe’s murder," a security source told the Irish Mirror.

“They are watching him 24/7 and are not letting him out of their sight.

“Gardai are just waiting on that one piece of the puzzle to nail these guys and they want to be ready to pounce once it does.

“Gardai investigating this brutal murder are adamant that those responsible will be brought to justice.

“He has a three-month visa for America but that is due to finish up soon. He will more than likely return to Ireland when it does but God only knows what he will decide to do.

“He is a loner in America at the moment after being shunned by all his close friends.

"And if he thinks things are bad for him now in America, he won’t know what hit him when he gets back to his home town.

“Everyone hates him for what he has done. He is not welcome back and will be run out of town if he does return.”

Meanwhile, the leader of the gang behind the murder was allegedly in a Co Monaghan nightclub over the weekend boasting that he was "the most feared man around" and would never be caught.

“He vanished for a while and everyone thought he’d skipped the country but he’s definitely back with a vengeance and is out every weekend acting like he runs the place," a source told the Irish Mirror.

“On Saturday night he was boasting about how he’s the big man and how no one will ever touch him because they know what would happen to them.

“It’s like he thinks he’s invincible, even without his motley crew, but people are hoping his big mouth gets him into trouble and he trips himself up soon.”