Detectives investigating the break-in at the Special Branch office in astlereagh station are seeking the extradition of a New York-based chef who they believe could be central to their inquiries.
Detectives from outside the Police Service of Northern Ireland (PSNI) are also being called in to "quality-assure" the investigation into the St Patrick's night raid.
The police commander in Belfast, Assistant Chief Constable Alan McQuillan, yesterday confirmed that detectives were preparing a file for the Northern Ireland Director of Public Prosecutions in connection with the proposed extradition.
The chef, whom police have already questioned in New York, worked at Castlereagh around the time of the theft of confidential files that included names and addresses of Special Branch officers and code names of their informants.
Senior police sources have insisted that the IRA was behind the break-in while Mr Gerry Adams and other leading Sinn Féin figures have categorically denied these claims.
Republicans have alleged a "dirty tricks" agenda behind the raid. Some Special Branch sources have also suggested that British military intelligence could be involved in a scheme to discredit the PSNI Special Branch so that MI5 might be given responsibility for the covert end of policing.
All security contacts agree that, irrespective of who instigated the theft, the files are in the hands of the IRA, a claim that Sinn Féin also denies.
The raid has led to much speculation and confusion, and for the PSNI much embarrassment. The Northern Secretary, Dr John Reid, announced a British government inquiry into the break-in, while Det Chief Supt Phil Wright was assigned to lead the PSNI investigation.
At Chief Supt Wright's request Mr McMillan agreed to call in nine officers from the London Metropolitan Police to oversee the inquiry.
