A gunman in a passing car shot a 52-year-old prison guard, David Black, as he drove to work Thursday on Northern Ireland's main highway.
Police have arrested three suspected Irish Republican Army militants, including Colin Duffy, a prominent Irish republican who has successfully defended himself against a series of murder charges dating back to 1993.
All three men remain in police custody.
U.S. Secretary of State Hillary Rodham Clinton is among those to have condemned the slaying, the first killing of a prison officer in the British territory since 1993.
Police said on Saturday that Black's funeral would take place Tuesday at Molesworth Presbyterian Church in Cookstown.