Some 700 classified files and 10,000 Surrey Police documents could be put before a coroner as part of the resumed Guildford pub bombings inquest.
A pre-inquest review heard Surrey coroner Richard Travers had written to the Home Office to access hundreds of closed files on the 1974 IRA blasts.
The hearing was told records may also be held by two other police forces and the Ministry of Defence (MoD).
Lawyers for a survivor and victim said searches should widen further.
Soldiers Ann Hamilton, 19, Caroline Slater, 18, William Forsyth, 18, and John Hunter, 17, died following the first blast at the Horse and Groom on 5 October, along with 21-year-old plasterer Paul Craig. A further 65 people were injured.
Eleven people - the Guildford Four and Maguire Seven - were wrongly jailed in what became known as one of Britain's biggest miscarriages of justice.
The pre-inquest hearing was told archives could extend beyond 700 files from a five-year inquiry by retired judge Sir John May into the wrongful convictions, to records created by other government departments including the MoD, and also papers held by the Met Police and Avon and Somerset Police.
Brenda Campbell QC, instructed by KRW Law for the family of victim Ann Hamilton and survivor Yvonne Tagg, said Surrey Police had already logged some 6,000 documents, but added: "It may be in the region of 10,000."
She told the coroner: "We would ask you to direct the MoD to communicate with you...so we know what if anything the MoD may have and the same goes for Avon and Somerset Police and for the Metropolitan Police."
Lawyers for the Met Police and the MoD said work was ongoing to discover what archives existed.
James Berry, for the Met, said the London bomb squad had responded to the explosions and seconded officers to the Surrey investigation, but added: "It may well be since they were working for Surrey, their documents may be held by Surrey."
Edward Pleeth, for the MoD, said the department was making inquiries, however "we simply don't know what exists".
Since 2016, the BBC has accessed several files from the May inquiry at The National Archives (TNA).
However, it was refused access to TNA's copy of the report by Avon and Somerset Police, which investigated Surrey Police's handling of the investigation, and when the BBC approached Avon and Somerset directly, it was told: "No information is held."