Four men and a woman were being questioned in connection with the murder of journalist Martin O’Hagan seven years ago.
They were detained in police raids on homes in Lurgan, Co Armagh, the area in which the reporter was gunned down, as well as neighbouring Banbridge, Co Down.
Nobody has ever been charged with the killing which was carried out by loyalist paramilitaries in Mid Ulster.
Mr O’Hagan, aged 51, was working for the Sunday World newspaper at the time and had written several articles exposing terrorist and drug-dealing activities by loyalists in the area where he lived.
He was shot dead in September 2001 as he walked home with his wife from a pub in Lurgan.
Detectives have questioned several suspects before and among those detained are believed to be two brothers. All five were detained on Tuesday, but details of the arrests only emerged last night.
The killing was claimed by the Red Hand Defenders, but its believed the men involved had links with the mid-Ulster based Loyalist Volunteer Force.