A coroner has ruled that certain information held by police and the security services relating to the Manchester Arena atrocity and bomber Salman Abedi is so sensitive it cannot be made public.
Sir John Saunders, who is presiding over an inquest hearing into the 22 deaths, said in a published ruling that revealing the evidence 'would assist terrorists'.
The inquest hearing could now become a public inquiry, which could enable the contested evidence to be heard behind closed doors rather than not at all.
The Home Secretary on behalf of the security services, MI5 and MI6, and Counter Terrorism Police North West, both submitted claims for public interest immunity (PII) over sensitive information in their possession which could relate to the scope of the inquest.
PII claims, if successful, lead to information and material not being disclosed or revealed in public legal settings.
Part of a pre-inquest review hearing held in Manchester last week took place in private to consider the applications over material related to the issue of whether the attack could have been prevented.
Legal representatives voiced fears over the claims at the hearing, saying the 'demands for transparency' should be 'at their highest'.
John Cooper QC, who represents a number of bereaved families, told the hearing that 'the very people who are seeking to restrict this material being placed in the public domain...are the very people who could be potentially severely criticised'.
The coroner, however, has now upheld both PII claims.
Mr Justice Saunders said: "I have done that because I am satisfied, having heard justifications for them, that to make public those matters would assist terrorists in carrying out the sort of atrocities committed in Manchester and would make it less likely that the security service and counter terrorism police would be able to prevent them.
"The balancing exercise strongly favours the material in question not being disclosed.
"I will, of course, keep this ruling under review."
Mr Justice Saunders said he couldn't set out the nature of the material in question.
But he said in the ruling: "The risk which is identified in each case is that disclosure of the information will make it easier for terrorists to kill people by avoiding detection before they are able to carry out an attack."
It's not yet known whether the inquest will become a public inquiry.
But Mr Justice Saunders added that, as a consequence of his PII ruling, his provisional view was that an 'adequate investigation' could not be conducted within the frameworks of the inquest.
It's thought that discussions of whether the inquest could be converted into a public inquiry will be raised at the next pre-inquest review hearing, scheduled for October 7.
Mr Justice Saunders reassured families attending last week's hearing at Manchester Town Hall that public interest immunity 'will not be used as a device for covering up responsibility' and that he would do his 'very best' to ensure that does not happen.
In Friday's ruling, the coroner said Home Secretary Priti Patel had submitted a PII certificate in which she stated it was her view that disclosing the material subject to her claim would 'damage national security'.
He added she had made it clear she was aware of the importance of the public interest in disclosure of all relevant matters within the scope of the inquests and that it was also important that justice should be carried out in public.
He said: "She states that she has carried out a balancing act between these two public interests and she is satisfied that the balance is in favour of non-disclosure.
"While she has set out her conclusions, the Secretary of State accepts that it is ultimately for me to carry out the balancing act."
The inquests are set to 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.
Abedi, who was 22, detonated a rucksack bomb in a foyer area of the Arena on May 22, 2017, after an Ariana Grande concert, killing 22 people and injuring many hundreds more.
It was the deadliest attack in the UK since the 2005 London bombings.
Abedi's brother, Hashem Abedi, 22, is charged with 22 counts of murder, attempted murder and conspiracy to cause an explosion likely to endanger life.
He's due to go on trial in November.
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