US Secretary of State Hillary Clinton has described the killing of David Black in Co Armagh as a senseless cowardly act.
In a statement, Mrs Clinton said there could be no justification for the murder of the prison officer who was shot as he made his way to work on Thursday.
Three men are being questioned in connection with the murder of David Black.
Two of the men, aged 31 and 44, were arrested in the Lurgan area of Armagh, while the other, a 29-year-old man was detained in Co Leitrim yesterday evening.
One of the men questioned is prominent dissident republican Colin Duffy.
Detectives have appealed for information about a Dublin-registered Toyota Camry used by the killers.
The dark blue car had a registration plate 94 D 5099.
Detective Superintendent Keith Agnew, who is leading the murder inquiry, has also appealed for the public's help in relation to Mr Black's car.
Police want to hear from anyone who saw his Audi A4, registration number HHZ 1560, at any stage during the drive from Cookstown, through Stewartstown, onto the Tamnamore roundabout and eastbound onto the M1.
Superintendent Agnew said this afternoon that there was "a long way to go" in this murder investigation.
Mr Black had over 30 years' service in the prison service and was nearing retirement.
His funeral will take place on Tuesday in Molesworth Presbyterian Church.