Two police forces have been asked if they received advance warning of the 1974 Birmingham pub bombings.
A pre-inquest review into the attack heard the Police Service of Northern Ireland and the Metropolitan Police, as well as HM Prison Service, had been written to.
Victims' families claim the state may have covered up its involvement in the bombings, which killed 21 people.
The inquest is due to start in September.
During separate hearings in 2016, senior coroner for Birmingham and Solihull Louise Hunt heard West Midlands Police had said it had no information relevant to any cover-up claims.
At the latest hearing, coroner Peter Thornton QC said the same question had now been put to the prisons and police services.
Peter Skelton, independent counsel for the coroner, said information was being sought about a claim, first aired last year, that the state may also have been protecting a government informant.
He said: "We have written to the Police Service of Northern Ireland, the Prison Service and the Metropolitan Police Service.
"We will await those answers in due course."
On November 21 1974, the IRA planted two bombs which ripped through two Birmingham pubs.
The police investigation into the attacks led to the wrongful convictions of the Birmingham Six.
Mr Thornton told the hearing that "further investigation work" would continue.
The hearings will include post-mortem examination reports and written summaries of the victims which will be read out to the inquest jury - similar to those read for the 96 victims during the Hillsborough inquiry.
A further pre-inquest review hearing will take place on May 31.