Thieves stole more than $39 million from the Belfast headquarters of Northern Bank (search) — the biggest robbery in Northern Ireland (search) history, authorities said Tuesday.

Police said the thieves took the families of at least two senior Northern Bank executives hostage Sunday at their homes, then forced the employees to help the gang gain access to the vault after the bank closed for business Monday night.

Police weren't informed about the robbery until nearly midnight, six hours after the gang began clearing the safe. They said one member of the Northern Bank employees' families was hospitalized with hypothermia in circumstances that weren't explained.

The targeted safe stored money being distributed to all 95 branches of Northern Bank in Northern Ireland ahead of Christmas, when banks typically handle their greatest volumes of cash. Police suspect the gang used a large truck to carry away the cash.

Northern Bank said it could not comment on any details of the raid. "However, initial indications are that affected staff are safe. This is our No. 1 priority," it said in a statement.

Police say more than 80 major crime gangs operate in Northern Ireland, and many gang members belong to illegal paramilitary groups. They declined to say Tuesday whether any particular group was suspected of mounting the Northern Bank raid.

The group with the biggest reputation for mounting bank robberies is the Irish Republican Army (search), which is observing a 1997 cease-fire but continues to run a range of criminal enterprises, including cigarette and fuel smuggling. The group in the past has gained access to high-security targets by taking the families of employees hostage.

Police in May accused the IRA of taking staff hostage at Northern Ireland's biggest retail superstore, then stealing more than $7.75 million in goods loaded onto a truck.

The raid came a week after Northern Bank's owner, National Australia Bank (search), announced it was selling the bank to Danske Bank (search) of Denmark in a deal expected to conclude in spring 2005.