One of the bidders for Nama's northern debt portfolio expressed concern in a letter to the Taoiseach's office over the business practices of third parties leading up to the £1.2bn sale.
Fortress Investment Group, one of the final three potential buyers alongside US rivals Pimco and Cerberus Capital Management, is understood to have sent the letter to the Department of the Taoiseach in February 2014.
The sale of the portfolio of 850 properties to New York-based Cerberus was announced in April last year.
The process, handled by advisers Lazard on behalf of Nama, had been formally kicked off in January 2014.
According to one person with knowledge of the letter to the Taoiseach's office, it complained about business practices leading up to the sale of Nama's loan book, dubbed Project Eagle.
It is unclear whether the same complaint was sent to Nama or whether the Taoiseach's office alerted Nama to the existence of the letter.
A Nama spokesman said: "We do not comment on correspondence to which Nama was not a party".
The Department of the Taoiseach and Nama did not respond to requests for information by the time of publication last night.
The integrity of the transaction was called into question in July this year after it emerged that a managing partner in leading Belfast law firm Tughans had transferred more than £7m linked to the Cerberus deal to an offshore bank account.
A portion of the money was understood to have been intended to facilitate payments to deal fixers.
Claims were made by Independent TD Mick Wallace that a Northern Ireland politician or party was to gain.
Authorities in the United States are stepping up their scrutiny of corruption allegations surrounding the disposal of Project Eagle.
The US Department of Justice (DoJ) is in the early stages of scrutinising the deal and has sent a subpoena for information to Cerberus.
The Irish News understands that Fortress has also been approached by the DoJ for information.
Fortress did not return calls and the DoJ did not respond to a request for information.
In a statement issued earlier this month, a Cerberus spokesman said that it had not been accused of any wrongdoing.
"As best we know, these matters are related to the alleged conduct of third parties and not to Cerberus or any of its affiliates," he said.
Timeline: January 2014: Nama officially kicks off Project Eagle sale February 2014: Fortress writes to Taoiseach voicing concerns over third party practices in the sales process March 2014: Pimco withdraw from bidding process April 2014: Cerberus is unveiled as the winning bidder
A bid by Pimco was aborted after it told Nama that a £5m payment was due to be made to businessman Frank Cushnahan if it succeeded in buying the portfolio.
Mr Cushnahan, who has denied any wrongdoing, had resigned as a member of the Nama Northern Ireland advisory just months before the Nama tender.
Tughans, the Belfast law firm where he had an office, and US law firm Brown Rudnick were also said to be in line for a £5m success fee each.
Both Tughans and Brown Rudnick went on to represent Cerberus in its bid.
Nama said that the final bid from Cerberus was £1.241 billion, with Fortress offering almost £1.1 billion.
Fortress has declined an invite to appear in front of the Assembly finance committee.
The committee, which is investigating the Project Eagle sale, is understood to have sent a letter with a series of questions to Fortress.
It has sought clarification on what representations, if any, were received in relation to bidding on the Project Eagle sale and on why it submitted a bid which was below the reserve price of £1.3 billion.
