Northern Bank officials have begun an audit to assess how much was stolen at the bank's headquarters in Belfast, amid fears that £20 million (€28.57 million) or more could have been taken in Monday's raid.
The police officer in charge of the investigation could not say whether any paramilitary grouping was associated with the meticulously planned robbery.
Assistant Chief Constable Sam Kinkaid left open the possibility that the robbery could have been carried out by a criminal gang with no paramilitary connections. "We don't know, it's a possibility. It could be paramilitary related."
He added: "This was not a lucky crime, this was a well-organised crime."
The raid is one of the largest robberies ever carried out in Britain or Ireland.
Dozens of officers from a range of PSNI branches are investigating it. Last night a police source said all lines of inquiry, including the possibility that the gang used inside intelligence, were being followed.
Gunmen arrived at the home of two senior Northern Bank officials at 10 p.m. on Sunday, one in Dunmurry just south of Belfast, the other at Loughinisland, a hamlet near Ballynahinch, Co Down.
According to a local source, two gang members posed as PSNI officers at the front door of Mr and Mrs Kevin McMullan's Loughinisland home. They said a relative of the McMullans had been injured in a road accident. When invited inside, the gang pulled a gun and threatened the couple.
Mr McMullan, who is in his early 30s, was ordered to leave for his office and to work normally throughout Monday until the staff went home.
It is understood a similar ploy was carried out by gang members at the home of the second senior official in Dunmurry at the same time.
Very large amounts of cash came into the bank throughout business hours on Monday, swelling the amount in the vault after one of the busiest weekends of trading in the year.
After other bank staff left the city-centre office, the two senior bank officials are understood to have been forced to facilitate the robbers gaining access to the vault. The money was transferred into one or, possibly, two large lorries.
The operation took two hours to complete before the cash, much of it in distinctive Northern Ireland banknotes, was driven off.
The vault would have held the takings from a number of businesses across Belfast, as well as the cash available for the Northern Bank's 95 branches and many more automated cash dispensers.
It is possible the robbers were surprised at the amount of cash stored in the bank's vault.
The two officials were released some time after 10 p.m.
According to Mr Patsy Toman, an SDLP councillor from the Loughinisland area, Mrs McMullan was freed in a remote forest at Drumkeeragh near Dromara, about 10 miles from her home. Last night, police were examining a burnt-out car at the scene. Although both families have suffered trauma, no-one was physically harmed.
There have been more than 40 so-called "tiger kidnappings" this year. They usually involve gunmen holding a hostage while other gang members take a family member to their place of work to help facilitate the robbery.
The Northern Bank robbery took more than 24 hours to carry out and may have involved up to 20 gang members in all three crime scenes. The raid is estimated to have been months in the planning.
The raid followed last week's announcement that the Northern Bank's owner, National Australia Bank, was selling the operation to Danske Bank of Denmark in a deal expected to be concluded next February.
The Minister for Justice, Mr McDowell, said he did not wish to prejudge whether paramilitaries were involved in the robbery. However, it "points to the danger of huge amounts of that kind (of money) falling into the wrong hands", he said.