Police commanders have unveiled a mammoth criminal investigation into a former Irish Republican Army double agent codenamed "Stakeknife," who was allegedly permitted by British Army intelligence officers to interrogate, torture and kill IRA colleagues to maintain his cover as the outlawed group's internal security chief.

Friday's launch of the Stakeknife probe at Northern Ireland police headquarters in Belfast featured a pledge from the investigation leader, Chief Constable Jon Boutcher from England's Bedfordshire Police, to follow the evidence wherever it leads his 50-detective team. The London-based investigation is expected to last five years and cost 30 million pounds ($43 million).

Former intelligence agents have described Stakeknife as Britain's most centrally placed mole within the IRA. British media identified him in 2003 as Belfast IRA veteran Freddie Scappaticci, who denied the allegations.