More than £20m may have been stolen in a raid at a bank's headquarters in Belfast, police have said.
Police say it is too early to say if paramilitaries were involved
The families of two Northern Bank executives were held hostage for 24 hours.
The robbery happened at the bank's headquarters in Donegall Square West in the city.
It is believed it could be one of the biggest cash robberies in the UK. The bank said a "significant" amount of money had been stolen.
It is understood the families of two executives of the bank, one in Loughinisland, County Down, the other in Dunmurry, near Belfast, were taken hostage on Sunday night.
It is thought the two officials went to work on Monday while their families were being held captive. They were released once the robbery was reported at about 2345 GMT on Monday.
It is understood that no-one was injured but one person was treated for hypothermia.
Police are investigating the possibility that a car found burnt out in Drumkeeragh Forest Park, between Ballynahinch and Castlewellan, may be linked to the robbery.
The building at the centre of the robbery houses the bank's cash centre, where tens of millions of pounds are believed to have been stored.
Retailers' Christmas takings and money to be distributed to cash dispensers for the last-minute shopping rush would be held in its underground vault.
A police team headed by Assistant Chief Constable Sam Kinkaid was immediately called in. Its officers specialise in investigating serious and organised crime.
Britain's Biggest Robberies
2002: $6.5 in cash stolen by gang from a security van at Heathrow Airport.
1990: £292m in bonds stolen in the mugging of a City financial messenger.
1987: Over £10m worth of goods stolen from a safe deposit box warehouse in Knightsbridge.
1983: £26m in gold bullion and diamond stolen in Brinks Mat robbery.
ACC Kinkaid said a well-organised criminal gang was involved.
He said it was "far too early" to say whether they had "any connections to a paramilitary group".
ACC Kinkaid added: "I have been informed that the sum involved is quite considerable and may be in excess of £20m."
Police officers took surveillance tapes from various cameras at the building, including ones outside the bank.
Detectives and uniformed officers are at the bank as the investigation continues.
In a statement on Tuesday, the bank said: "A theft has occurred at Northern Bank in Belfast, a wholly-owned subsidiary of National
It is understood two employees of the bank were held captive
"The theft is being investigated by the police in Northern Ireland and we cannot discuss
details at this stage.
"However, initial indications are that affected staff are safe - this is our number one
The external access point to the building from Wellington Street was sealed off on Tuesday but business went on as usual at the headquarters.
Earlier this month, the Northern Bank and the National Irish Bank were sold to Denmark's biggest bank, Danske.
The Northern Bank has 95 branches and 30% of the market in Northern Ireland.