The agency leading the inquiry into the £1.2 billion sale of NAMA's Northern Ireland property portfolio says eight people are now under criminal investigation in connection with the sale.
The National Asset Management Agency sold the loan portfolio to a US investment fund in 2014.
The UK National Crime Agency took over the investigation into fraud, bribery and corruption allegations in July 2015.
It said more than 60 witnesses had now been questioned.
The NCA said three people have been arrested and hundreds of thousands of documents have been seized.
Details of the investigation were revealed in an interview with the BBC by the director of the NCA, Lynne Owens, during a visit to Belfast.
She said they have eight potential suspects under investigation, seven of whom have already been interviewed
Ms Owens said they have taken 61 witness statements and applied for six court production orders with another one in train.
She said they have also made three international letters of request to the Isle of Man and the Republic of Ireland because this is an international investigation.
Ms Owens said those letters were seeking what she said were "any intelligence or evidence to prove or disprove allegations of bribery, fraud etc".
The agency said it cannot comment at this stage on whether it has uncovered evidence of criminal activity.
It was revealed in court proceedings earlier this year that the NCA told a businessman linked to the NAMA investigation that it hoped to complete it by June.
But that did not happen and Ms Owens said it was impossible to put a definitive timescale on the investigation.
She said the NCA has seized hundreds of thousands of pieces of paper and bits of information that now need to be fully examined.
"We follow the evidence wherever it takes us. This is an investigation that covers a seven-year period with vast quantities of material recovered, some of that requires forensic accountant work to be done on it, and those things take time," said Ms Owens.
"We are doing everything as thoroughly as we possibly can and we will get to the stage where we do put a file to the Public Prosecution Service to consider."