A 50-year-old man from Carrickfergus, County Antrim, has been arrested by detectives investigating the £26.5 million Stg Northern Bank robbery in Belfast last December.
The arrest came as officers continued to detain one of the hostages taken during the audacious raid just before Christmas when the vaults of the Northern's HQ were cleared.
The bank official, who has been off work on sick leave since the robbery, was arrested along with a 22-year-old woman on Tuesday.
She was later released without charge but detectives were given a three-day extension to continue questioning the bank's employee about the robbery.
Police chiefs on both sides of the border have blamed the IRA for carrying out the raid, which plunged the Northern Ireland peace process into crisis at the time.
Three men have already been charged by the team of detectives probing the raid - one with the robbery itself.