Families of those murdered in the Manchester Arena suicide bombing have been promised a "rigorous" investigation into the police and security services, a court heard.
Coroner and retired High Court judge Sir John Saunders said the families will have to "take his word" that he will investigate the authorities thoroughly during a hearing where lawyers for the authorities applied for evidence to be kept secret in the forthcoming inquests.
Lawyers for the families said the police and security services applying for public interest immunity (PII) for some evidence on the grounds of national security were the same people who may be in the "firing line" for criticism during the inquests.
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The hearing, the fourth pre-inquest review hearing, was held at Manchester Town Hall ahead of full inquests next April into the deaths of the 22 people murdered by suicide bomber Salman Abedi, 22, at the end of an Ariana Grande concert at Manchester Arena on May 22 2017.
The inquests will examine the build-up and the attack itself, security at the arena, the emergency response and the victims and their cause of death.
They will also look at whether the attack could have been prevented and the role of the police and security services.
Sir John said: "This will be a vigorous process. Public interest immunity will not be used as a device for covering up responsibility and I will do my very best that does not happen. A rigorous investigation will take place."
Part of the hearing was held in private for the coroner to consider the PII applications by the Home Office and Greater Manchester Police (GMP).
Unusually, the coroner and lawyers representing GMP and the Government were at an undisclosed location, with proceedings live-streamed from there to the town hall where lawyers representing the families and press were present.
The two-hour public part of the hearing heard lawyers for the families and authorities dispute the relative importance of public security and the necessity of public justice.
Sir John must rule whether to allow or reject the applications by the authorities.
Sir James Eadie QC, representing the Home Office, said while public, open justice was a "thoroughly important matter", he suggested the balance lay in favour of not making public information about the police and security services that may be of use to terrorists, so national security held the "whip hand".
He said: "National security, if you will, is at the very top of the tree in terms of protection of the public and, therefore, public interest."
Alan Payne QC, representing GMP, said the evidence in the application had been analysed and their counter-terrorism experts concluded it could not be made public without jeopardising national security.
However, lawyers for the families of the 22, who have not seen the evidence the authorities want to withhold, stressed the need for as open an inquest process as possible.
John Cooper QC, representing a number of the families, told the coroner: "When you consider material that will be placed before you, we submit you should bear in mind the very people seeking to restrict material being placed in the public domain, the people making the application are the very people who could potentially be severely criticised, and the ramifications of that are significant.
"There are many in this court that have been in cases where national security has been cited, particularly in military inquests, when perhaps it's not national security that's the concern, but national humiliation, in the sense of matters being placed before the court that's embarrassing to services."
Pete Weatherby QC, also representing families, said: "There's fear from the families that the public interest immunity applications are being cast too wide. We urge you to take a very anxious approach to the scrutiny of these matters."
The coroner, counsel to the inquest and lawyers for the Home Office and police later went into private session to discuss the detail of the PII applications.
Sir John's decision will be made public at a later date.