Lawyers have questioned the status of the Surrey Police inquiry into the Guildford pub bombings after seeing fresh evidence of a "failure to act".
Four people were wrongly jailed for the 1974 attacks. An IRA cell later claimed responsibility but was not charged.
Belfast lawyer Christopher Stanley said evidence seen at the National Archives that police never acted on the confessions needed to be investigated.
Surrey Police said the inquiry was not active but new leads would be reviewed.
Mr Stanley said: "We're asking for the status of any inquiry, despite the fact they said on the record it's closed."
Adding that there was "no closure in a murder investigation", he said: "There are still unanswered questions, there are still questions of accountability, there are still questions of state failing and these things don't have an historical time limit placed on them."
In a statement, Surrey Police said: "The Guildford pub bombings which took place in 1974 are not currently subject to an active investigation.
"However, should any new or fresh information come to light, this will be reviewed.
"Surrey Police co-operated fully with the public enquiry by Sir John May in 1994 and the release of any files relating to this enquiry would be a matter for the Home Office."
About 50 files from a controversial inquiry into the miscarriage of justice are open at Kew, but more than 700 remain closed.
KRW Law has begun acting for Ann McKernan, sister of Guildford Four member Gerry Conlon, after the BBC obtained six files of working papers under freedom of information legislation.
The firm believes there was an attempt to tarnish Mr Conlon's reputation.
Mr Stanley said lawyers would examine the open files and ask to see the remaining hundreds of closed files.
Transcripts of private, oral hearings carried out by Sir John May in his five-year probe into the convictions are open at the Public Record Office.
In one hearing, Sir John questioned senior police officers and said "the $64,000 question" was what steps Surrey Police took after the Balcombe Street IRA unit accepted in 1975 it had been involved.
Lawyers have also requested access to the original inquest papers.
Guildford Four timeline
- 5 October 1974 - IRA bombs explode in two pubs in Guildford, Surrey, killing five people and injuring scores more. Guildford was known as a "garrison town", with several barracks nearby, at Stoughton and Pirbright and Aldershot in Hampshire, and a night-life that was popular with the 6,000 military personnel in the area
- 22 October 1975 - Paul Hill, Gerry Conlon, Patrick Armstrong and Carole Richardson - the Guildford Four - jailed for life at the Old Bailey
- 19 October 1989 - After years of campaigning, the Court of Appeal quashes the convictions, ruling them as unsafe, and releases them
- 9 February 2005 - Prime Minister Tony Blair formally apologises to the Guildford Four for the miscarriage of justice they suffered
- 21 June 2014 - Gerry Conlon, who demanded the release of the files, dies aged 60