The On The Runs (OTRs) outside the jurisdiction received messages telling them they were not currently sought by police, although future prosecutions were not ruled out.
They are alleged to be behind some of Britain’s worst attacks.
The British 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 reports, 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 Féin.
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 Féin 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 British 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 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.
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.
On Friday, the Police Service of Northern Ireland (PSNI) wrote to MPs on the affairs select committee to disclose details of the major new phase in its inquiries into the OTR cases, according to reports.
In the letter, Will Kerr, the Assistant Chief Constable, announced the PSNI now believes there to be no barrier to prosecuting OTR suspects who had been sent “comfort letters” — because UK ministers recently said the letters had no legal force.
“Whilst prosecutorial challenges may still lie ahead, the obstacles to investigating, arresting, and charging suspects appear to have been removed,” he said.
The PSNI is to “prioritise” re-examining the cases of 36 OTRs whose status was changed from “wanted” to “not wanted” during the scheme. It is not clear if any of these 36 individuals were wrongly told they were not wanted, but police want to review the cases thoroughly to establish the truth.
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