DUBLIN — Northern Ireland police arrested three suspected Irish Republican Army militants Friday on suspicion of killing an off-duty prison officer, a rare slaying that has inspired political condemnation across Britain and Ireland.

The three men aged 29, 31, and 44 were arrested at their homes in Lurgan, a power base for IRA diehards opposed to Northern Ireland peacemaking. Police said the elder suspect is Colin Duffy, the most prominent Irish republican in Lurgan, who has successfully defended himself against a series of murder charges dating back to 1993.