A 71 year old man, arrested in relation to the shooting of police officer John Caldwell, has been released from custody.
The detective chief inspector remains in a critical condition in hospital after being shot in front of his son in Omagh, County Tyrone last week.
The New IRA remains the PSNI's main line of inquiry.
Three other men - aged 33, 47 and 57 - are still being questioned.
A 20 thousand pound reward is being offered by the charity Crimestoppers for information that leads to the conviction of those behind the shooting.
