Northern Ireland police chief Hugh Orde has been updating the Policing Board on the £26.5m Northern Bank raid.
Chief Constable Hugh Orde has blamed the IRA for the raid
The IRA has twice denied it was behind last month's robbery in Belfast but Mr Orde's assessment has not changed.
The BBC understands that the police believe the robbery was planned and carried out by the IRA in Belfast.
Police have a list of suspected IRA figures they believe were involved, names which have been included in security briefings given to government.
The chief constable's meeting with the Policing Board, which monitors the activities of the police service, took place behind closed doors on Thursday.
Speaking after the meeting, the SDLP's Alex Attwood said the chief constable had provided further confirmation that his assessment of Provisional IRA involvement was correct.
"The chief constable brought the Policing Board into his confidence on a range of matters, in a way that quite surprised me," he said.
"On the basis of what he confirmed to the board today, I, and Joe Byrne in the SDLP, have no doubt, whatsoever, that his attribution in relation to this matter is correct, and that this inquiry is proceeding absolutely properly."
He said the Garda Siochana in the Republic of Ireland were co-operating in the inquiry.
Sammy Wilson of the DUP also confirmed that the board members accepted the chief constable's view.
However, Sinn Fein's Gerry Kelly said the onus was on Mr Orde to provide the evidence to back the "groundless allegations".
"People politically are jumping on the bandwagon here to try and beat up on republicans," he said.
"It's not as if they are objective in the situation. This comes from a single funnel of intelligence out of the PSNI."
BBC Northern Ireland security editor Brian Rowan said he had learned that the police assessment was based both on intelligence information and on evidence that had emerged since the robbery.
He said that the police believe the raid on the bank's head office in Belfast on 20 December was carried out under the direction of one of the IRA's most senior leaders in the city.
It is also suggested that they had help from elsewhere in the IRA organisation.
Paul Murphy imposed financial sanctions on Sinn Fein in April
This week, the IMC (Independent Monitoring Commission), which reports on paramilitary activity, has been meeting in Belfast to consider what it should do.
Its next report is not due until April but an earlier assessment is expected.
BBC Northern Ireland political editor Mark Devenport said that an assessment by the IMC, which concludes that the IRA was behind the raid, would increase the calls for Sinn Fein to be excluded from the political process and for sanctions to be imposed on the party.
However, he said that the British and Irish governments will probably resist calls for sanctions to be placed on Sinn Fein and the IRA until after the two prime ministers meet at the beginning of next month.
A previous financial penalty imposed on Sinn Fein by the Secretary of State, Paul Murphy, is currently being challenged at the High Court in Belfast.
The party lost £120,000 last April after the IMC reported that the IRA was involved in criminal activity.
Delivering Sinn Fein's application for a judicial review on Thursday, Michael Lavery, QC, argued that the IMC was not an appropriate body to take decisions which would lead to the taking away of rights from parties and individuals.
Papers lodged in support of the application stated: "Sinn Fein is not the IRA and is publicly committed to exclusively peaceful and democratic means."
Mr Lavery submitted that the IMC had not shown any evidential connection between the alleged activities of the IRA and Sinn Fein.