Detectives were given more time tonight to question a Northern Bank staff member about the £26.5m (€38m) robbery last December.
Chris Ward, 24, from Poleglass, West Belfast, has been in custody since his arrest a week ago.
Police were granted another 48 hours to hold him, a spokesman said tonight.
Mr Ward was held hostage by the gang which carried out the robbery on the bank’s headquarters in the centre of Belfast just before Christmas.
Detectives investigating the robbery have already charged three men, one of them for carrying out the raid.
The Chief Constable Sir Hugh Orde has blamed the IRA for robbing the bank.