The Police Service of Northern Ireland has refused to be drawn on the question of paramilitary involvement in Monday's robbery at the Northern Bank in Belfast.
Mr Sam Kinkaid, head of the PSNI branch that investigates organised crime, said yesterday it was "far too early" to speculate on which paramilitary group, if any, was responsible.
He also rebuffed questions about a possible cross-Border link.
"We have had a number of organised crime gangs working here in Northern Ireland who have targeted banks and bank officials and other financial institutions, who have not had a connection with paramilitary groups. But equally we have had a number of paramilitary groups who have specialised in house take-overs and the types of crime that happened at the Northern Bank.
"We don't know, it's a possibility. It could be paramilitary related," Mr Kinkaid said.
He described the theft as "a very well-organised" rather than a "lucky" operation, which could have taken months in the planning.
A large number of criminals would have been needed across the three crime scenes - at the homes of the two senior bank officials and at the city-centre bank, he added.
Mr Kinkaid said a fingertip forensic examination at the bank would prevent the bank from establishing just how much was stolen.
"We have had to hold the scene at the bank today to ensure a professional forensic examination is carried out," he said.
"That has somewhat delayed the bank's ability to carry out an audit and to give me an exact sum.
"But on the information that I have received to date from interviewing witnesses and from people who have been at the bank since early this morning, I have been informed that the sum is quite considerable and may be in excess of £20 million."
Mr Kinkaid said the need by the thieves to disperse or launder at least £20 million in distinctive Northern Ireland banknotes, which are virtually unusable elsewhere, would pose a problem for the gang.
"That is a large sum of money to try to spend or to circulate within Northern Ireland. It is an important point to raise in relation to members of the public. Anyone who clearly is in receipt of large sums of money or has been able to pay off bills it's very important that they [the public] contact detectives."
He promised to release further details as soon as the examination at the bank was completed.
"We are hoping to come back within the next few days with further information that will help us educate the public as to the nature of the sum that has been taken, the types of banknotes and what's in circulation."
Last night detectives were still examining footage from the bank's surveillance cameras and interviewing witnesses to the crime.