At least half-a-dozen agents from the US Department of Homeland Security have been in Ireland for several weeks, working with gardai on the investigation into the murder of Detective Adrian Donohoe.
The probe is continuing at an "intense pace" despite a 27-year-old man having been charged at Dundalk District Court on Sunday night with the officer's murder in January, 2013.
"The Department of Homeland Security has been liaising with gardai for a number of years in this case because some of the suspects have spent time in the United States," a senior source said last night.
"Two male suspects are curr- ently based in Boston, and these individuals are certainly persons of interest to Homeland Security because of the information gardai have provided.
"The department will continue to liaise with gardai for as long as these suspects remain on American soil."
A garda spokesman confirmed that representatives of US Homeland Security Investigations, led by acting deputy executive associate director Alysa D Erichs, met with acting Garda Commissioner Donall O Cualain yesterday in Dundalk Garda Station.
"Homeland Security Investigations has provided invaluable assistance to An Garda Siochana during the investigation into the murder of Detective Garda Adrian Donohoe," the spokesman said. "The investigation continues, and we again appeal to the public for their assistance."
Over the past nine months, the Department of Homeland Security has distributed posters in Boston and New York in which they appealed for information about the murder of the garda.
They have also set up a dedicated phone line in the US for people with information to contact them.
"There are expected to be more arrests in this case in the coming months," a source told the Herald.
"Co-operation between gardai and Homeland Security is very important as this investigation continues."
Last June, in New York, posters began appearing in Irish neighbourhoods of the city, urging those with information to come forward.
"Investigators believe people living in the Woodlawn/Yonkers/The Bronx area of New York may have information that could assist this investigation," the sign read.
"A substantial reward is on offer for information leading to the arrest and conviction of the persons involved."
The reward for credible information leading to a conviction is €100,000, or roughly $112,000. It emerged last month that federal agents and local police in Boston have increased pressure on two of a five-man gang living in the area and have offered a substantial reward for anyone who can provide information to help them.
The FBI and police in the US have specifically appealed for information from the Irish community.
Det Gda Donohoe, who was a father-of-two, was shot dead during an armed robbery at the Lordship Credit Union near Dundalk, Co Louth, on January 25, 2013.
He had arrived at the scene with Det Garda Joe Ryan to act as an armed escort for a staff member who was depositing a bag of money in a night safe.
A close older associate of two suspects living in the US was arrested as part of the investigation last Monday night before being released without charge.