Clinton condemns prison officer killing
The murder of prison officer David Black was an outrageous and cowardly act, United States Secretary of State Hillary Clinton has said.
The 52-year-old was shot on the M1 on Thursday morning as he drove to work at Maghaberry jail.
Mrs Clinton said the United States was resolute in its support for Northern Ireland's embrace of peace and reconciliation.
Mr Black's funeral is due to take place on Tuesday.
Three men are still being questioned in connection with the murder.
Irish police arrested a 29-year-old man in County Leitrim at 17:00 GMT on Friday. He is being questioned at Carrick-on-Shannon Garda station.
Earlier the PSNI arrested two men, aged 31 and 44, in Lurgan, Co Armagh. One is Colin Duffy, a prominent republican. They are being questioned in Antrim.
The Crimestoppers charity has offered up to £10,000 for information leading to the conviction of those responsible for the 52-year-old's murder.
Mr Black, from Cookstown in County Tyrone, was driving to work when a car pulled up beside his vehicle on he motorway in County Armagh and fired a number of shots. His car veered into a ditch and he died at the scene.
On Friday, the senior officer investigating the murder appealed for information about the movements of Mr Black's car and that of his killers.
Mr Black was driving a black Audi A4, with the registration number HHZ 1560.
Officers want to hear from anyone who saw it after 07:00 GMT on Thursday when it travelled from Cookstown, through Stewartstown and onto the Tamnamore roundabout before going onto the M1 eastbound.
The killers were driving a dark blue Toyota Camry which had an Irish registration number, 94 D 50997. It was found burnt out in Inglewood in north Lurgan at 07:50 GMT.
Mr Black, a father-of-two, was the first prison officer to be murdered in Northern Ireland in almost 20 years.
The Police Service of Northern Ireland (PSNI) said they believed dissident republican paramilitaries were responsible for the attack.
He was the 30th prison worker to be murdered by paramilitaries since 1974 in Northern Ireland.
A book of condolence opened in Mr Black's home town, Cookstown, on Friday at the Burnavon Arts and Culture Centre on Burn Road.