The Police Service of Northern Ireland and the Met Police have been asked for any information that they had advanced notice of the 1974 Birmingham pub bombings.
A pre-inquest review into the terrorist atrocity was told that three agencies including HM Prison Service had been written to, in order to ask about victims' families claims the state may have "covered-up" its involvement in the double bombings.
At the hearing on Thursday, coroner Peter Thornton QC also said he wanted the fresh hearings into the deaths of the 21 people killed to start in "late September" 2017.
During separate hearings last year, senior coroner for Birmingham and Solihull Louise Hunt heard that West Midlands Police had said it had no information relevant to any cover-up claims.
It emerged on Thursday that that same question had now been put to the prisons and police services.
Peter Skelton, independent counsel for the coroner, told the packed hearing in Birmingham that information was being sought about a claim, first aired last year, that the state may also have been protecting a government informant.
He said: "At the same time in order to ensure the questions were directed to all appropriate bodies of the state, not just West Midlands Police, we have written to the Police Service of Northern Ireland, the Prison Service and the Metropolitan Police Service, along similar lines, asking similar questions.
"We will await those answers in due course."
On the night of November 21 1974, the IRA planted two bombs which ripped through the Tavern in the Town and nearby Mulberry Bush pubs, killing 21 and injuring 182 others.
The botched police investigation into the attacks led to the wrongful convictions of the Birmingham Six - one of the most infamous miscarriages of justice in British legal history.
West Midlands Police opposed the resumption of fresh inquests, despite fresh evidence coming to light that police may have allegedly ignored two tip-offs of an imminent IRA attack in the city.
Mr Thornton told the hearing, packed with the victims' families, that "further investigation work" would also continue.
More information about the possible shape of the forthcoming inquests, which will be heard in front of a jury, was discussed.
The hearings will include 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.
Also included in evidence will be what the coroner called the "distressing" post-mortem examination reports.
A website is being set up to keep the public up-to-date on the hearings' progress.
The scope of the new inquest, and decisions on which experts and witnesses should be called, will be determined at a further pre-inquest review hearing in Birmingham on May 31.
Mr Thornton also directed that the hearing would be a Middleton - or Article 2 - inquest, under the convention on human rights, which can establish not only how the victims met their deaths but also the surrounding circumstances.
He said: "I am minded to say it will be an Article 2 inquest, not least on the advance notice issue identified by the senior coroner (Louise Hunt), that Article 2 is engaged."
The victims' families had been calling for just that type of inquest as the scope of what evidence can be heard is broader.