A man has been arrested in connection with the van bomb left on the Belfast to Dublin Road near Newry a week ago.
The 26-year-old was arrested in the Rostrevor area and is being questioned in Antrim.
The 500lb bomb was discovered in a van abandoned on the Newry bypass last Thursday.
Hundreds of motorists drove past the vehicle after cones cordoning it off were removed.
Army bomb experts carried out several controlled explosions on the vehicle.