A suspected Irish cop killer has fled to the United States where he entered the country illegally, Irish police say. The FBI and other US law enforcement agencies have been asked to help seek his apprehension and deportation.

The man in his twenties is the chief suspect in the gunning down of Garda Detective Adrian Donohoe, who was murdered in cold blood during a botched robbery near the Northern Irish border, in County Louth, in January.

He is described by police as being "of immense interest to us", and had been due on court on an unrelated matter when he fled, reports the Irish Independent.

He failed to appear and is believed to have fled to the east coast of the United States. A  bench warrant has now been issued for his arrest.

Adrian Donohoe, 41, was a  highly respected detective who was murdered outside a credit union when he tried to stop a robbery. He is survived by two young children and his wife, also a police officer.

The main suspects are a cross border criminal gang who originally operated out of Dublin.

The 41-year-old father-of-two was shot in the head as he approached an armed gang during the robbery.

The getaway car was later found burned out across the border in a forest in south Armagh.

Because the case involved police in two jurisdictions it has become complicated to pursue.

Police believe there were five members of the gang, all in their late teens and early twenties.

Police Commissioner Martin Callinan told a press conference progress on the case will  continue to be "slow and steady."

"The investigation is being very, very carefully worked through, it's very painstaking."

Meanwhile U.S. authorities will be asked to locate and deport the suspect as a matter of great urgency, say Irish police.

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