Police are reportedly investigating six key IRA suspects who received letters telling them they were not wanted.
The On The Runs (OTRs) outside the jurisdiction received messages telling them they were not currently sought by police but not ruling out future prosecutions.
The Sunday Telegraph said they were alleged to be behind some of Britain's worst attacks.
The Government has said it no longer stands by the letters, drawn up under Tony Blair's administration as a peace process move.
It followed the collapse of the prosecution last year of John Downey for the Hyde Park bombing which killed four soldiers in 1982.
According to the Sunday Telegraph, officers have found "six additional individuals whose circumstances have been identified as similar to the Downey case".
The leaked police document shows that detectives now believe these "comfort letters" do not protect suspects from prosecution.
It discloses that police in Northern Ireland have identified six individuals suspected of carrying out terrorist attacks.
Mr Blair's Government sent about 200 letters to republicans assuring them they were not being pursued by the UK authorities following requests from Sinn Fein.
Mr Blair began the peace process scheme in 2000 which saw 95 of the so-called letters of comfort issued by the government to suspects linked by intelligence to almost 300 murders.
The plan was drawn up following pressure from Sinn Fein to allow the fugitives, who had they been in prison before 1998 would have been released under the Good Friday Agreement, to return to Northern Ireland.
An investigation was launched by MPs when the prosecution of Mr Downey for the murder of the soldiers at Hyde Park in 1982 was halted after he received one of the OTR letters in error when he was wanted by the Metropolitan Police.
Mr Blair apologised to Hyde Park victims but made no apology for sending letters to those who should have received them.
He took responsibility for the structural problems with the scheme, which developed from a small to a large number of cases over many years, and said the framework could have been better. A judge said had the issues been corrected they may have prevented the Hyde Park error.
Household Cavalry Lieutenant Anthony "Denis" Daly, 23, died in the explosion alongside Trooper Simon Tipper, 19, Lance Corporal Jeffrey Young, 19, and 36-year-old Squadron Quartermaster Corporal Roy Bright.
Mr Blair said the letter should not have been issued to Mr Downey.
ACC Will Kerr said: "The review of these cases is an ongoing process. This does involve active criminal investigations and it would therefore be inappropriate to comment further at this stage."