The Stormont finance committee investigating the £1bn sale of Nama's Northern Ireland loan book to US company Cerberus is seeking legal advice over whether it can compel the department's chief civil servant and Nama to appear before it.

The inquiry into what the North's department of finance knew about the sales process hit a stumbling block on Monday when DUP minister Arlene Foster said its permanent secretary, David Sterling, would not appear before the committee again as planned and that she would have to consult with police before sharing any information, in an effort not to compromise an NCA (British National Crime Agency) criminal investigation.

The Stormont inquiry comes after independent TD Mick Wallace raised concerns during leaders' questions last month about the Nama sale of the Northern Ireland loan portfolio, Project Eagle, which involved more than 850 properties. Mr Wallace made allegations that Stg £7m has been "earmarked" for a Northern Ireland politician.

The committee expressed its disappointment with Stormont’s finance and personnel department when it met on Wednesday morning to decide the next steps in the review of Nama assets in Northern Ireland.

The Sinn Féin chairman of the committee Daithi McKay accused the department of “dragging the NCA into a political squabble” and said its refusal to allow the permanent secretary to appear and release papers is “unsatisfactory”. He claimed the NCA investigation should not be used as an excuse not to answer questions. “We need to get on with our business and if it requires using Section 44 powers then we need to use them,” Mr McKay said.

The committee agreed to publish Mrs Foster's response to its requests on the Assembly website, along with recent communication from Nama and Minister for Finance Michael Noonan.

Committee chairman Daithí McKay said letters from former DUP finance ministers Sammy Wilson MP and Simon Hamilton MLA indicated they were willing to assist its inquiry but had requested sight of the committee's legal advice received to ensure the NCA investigation would not be compromised.

Mr McKay spoke of a letter Sammy Wilson had written to Mr Noonan and suggested writing to Brown Rudnick as part of its plans to ascertain what the nature of any contact with former finance ministers was and when it began.

Independent MLA John McCallister said to hide behind the NCA was “totally unacceptable” and redacted documents from the department of finance “only fuels speculation”.

The committee hopes to meet again next week but its clerk advised it could take at least 10 days to prepare legal advice.

Sinn Féin MLA Mairtin O Muilleoir said the committee could "press on" and "restore faith of ordinary people", adding there were "matters of truth in relation to this dirty deal we can get to" without impinging on the NCA investigation.

“Some of the questions around why ministers in this government were writing to Mr Noonan recommending a sale of the Nama assets to one venture captial, or vulture capital company, why appointments were made, meetings between key individuals going back several years and our government ministers. They are things that can be answered and shed light on how governemnt dealt with Nama and private companies who wanted to purchase the Nama assets and our role in that.”

He described the department of finance’s refusal to hand over information as a “convenient catch all” and said he hopes former DUP finance minister Sammy Wilson will appear and not “hide behind this fig leaf of the NCA”.

Mr Ó Muilleoir branded the department’s “Dear John” letter earlier this week as “stinking”.

DUP Jim Wells said the department not wanting to prejudice any potential future court case was a very strong defence and that independent advice for the committee was "not to touch this with a bargepole", which was disputed by his committee colleagues.

SDLP MLA Dominic Bradley said the committee had confined itself to gathering factual information and would not pass value judgements so he could not see why there should be any legal difficulties around that.

Following the meeting Mr Bradley said: "The finance minister has attempted to frustrate the Committee investigation into the Project Eagle sale by refusing to allow officials to give evidence.

“The excuse of an NCA investigation is hollow and extremely transparent, especially given the fact that the permanent secretary has already appeared before the committee and gave an undertaking to return and provide information which he did not have to hand at that time.

“The committee’s terms of reference are robust and ensure that it will not endanger any possible future legal proceedings. The minister knows that, as do Simon Hamilton and Sammy Wilson, two former DUP finance ministers who have refused to appear before the committee.

“I am strongly of the view that the finance committee should use all the powers at its disposal to ensure that all witnesses with a contribution to make to this review are called and give evidence.”

A statement from Mr McKay on Wednesday said: “At today’s meeting of the finance committee inquiry into the sale of Nama’s northern portfolio we agreed in principle to use the committee’s power to compel witnesses to come to the inquiry.

“It was agreed to compel both Nama and the permanent secretary of the department of finance and personnel to attend the inquiry.

“The committee also agreed to invite Pimco to come to the inquiry to answer questions on its involvement in the transactions.”

On Tuesday in Dublin Sinn Féin called for a detailed review of Nama’s loan book to ensure it is bringing maximum value to Irish citizens.