The head of MI5 said he was “deeply sorry” that the security service did not prevent the Manchester Arena attack.
A public inquiry found that MI5 missed a significant chance to take action that might have stopped the 2017 bombing.
Chairman Sir John Saunders said the intelligence could have led to suicide bomber Salman Abedi being followed to a car where he stored his explosives.
MI5 Director General Ken McCallum said he regretted that such intelligence was not obtained.
“Gathering covert intelligence is difficult,” he said, “but had we been able to seize the small chance we had, those affected might not have experienced such terrible loss and trauma.”
Twenty-two people died and hundreds were injured when Abedi detonated his homemade device in the foyer of the Manchester Arena as crowds left an Ariana Grande concert on 22 May 2017.
The inquiry found that two pieces of information about Abedi were then assessed by the security service as not terrorist-related.
One officer admitted they considered a possible urgent national security issue on one of them, but did not immediately discuss it with colleagues and did not write a report the same day.
In his 207-page report, Sir John said: “The delay in delivering the report missed an opportunity to take potentially important investigative action.
“Based on everything the security service knew or should have known, I am satisfied that such investigative action would have been a proportionate and justified step to take.
– This should have happened.
But he added that Abedi “demonstrated some security awareness and that this may have affected the effectiveness of the investigative action that I have identified”.
Sir John said the intelligence could have led Abedi to be followed to the parked Nissan Micra where he stored his explosives and later moved them to a rented flat in the city center to assemble the bomb.
He said that if MI5 had acted on the intelligence received, Abedi could also have been stopped at Manchester Airport when he returned from Libya four days before the attack.
The public inquiry also found that Abedi was probably assisted by someone in Libya, but it was not possible, based on the available evidence, to say who this might have been.
It is the first time an official conclusion has been made about the possible involvement of other people from abroad.
In making this finding, Sir John contradicts an MI5 assessment which said that none other than Salman Abedi and his brother Hashem were knowingly involved in the plot.
The report also found that while the Didsbury mosque in south Manchester, where the Abedi family worshipped, was not an active factor in the brothers’ radicalisation, politicization did occur there.
Sir John said “there was a form of willful blindness” to some activities and “weak management”.
The report concluded that the Abedi family bore “significant responsibility” for the radicalization of Salman and Hashem Abedi.
The family members responsible include their father Ramadan Abedi, mother Samia Tabbal and elder brother Ismail Abedi, each of whom has held extremist views, the inquest found.
But Sir John said, apart from Hashem Abedi, there was insufficient evidence to attribute specific knowledge of the attack to them.
The first volume has been made publicly available while the second has only been circulated to a limited readership of people with security clearance.
This does not include the families of those who died, their legal teams or the media, as the content would be harmful to national security if made public, the inquest was told.
The investigative process began more than three years ago, and there were 194 days of oral evidence from 267 witnesses.
Keeping some findings private seems like a contradiction in terms of the term “public investigation”, but the chairman says he had to balance the principle of open justice with the issue of national security.
He has added that all private evidence has been scrutinized to ensure that restricting it cannot be seen as a “cloak to cover up mistakes”.
Given that the two previous inquiry reports were highly critical of the private companies and public authorities involved in the arena and preparedness, this document was also expected to be tough on MI5.
Bereaved families and survivors will be glad that the report does not hit home in some areas.
They will find it upsetting to hear that the security service missed a significant opportunity. But there will also be some frustration that they will never learn all the details of exactly what that opportunity entailed.
Andrew Roussos, whose eight-year-old daughter Saffie-Rose was killed in the blast, said hearing how this tragedy could have been avoided was “devastating for all of us”.
“This was a catastrophic failure… In my view, the fact that MI5 failed to stop him despite all the red flags available shows that they are not fit to keep us safe and therefore not fit for purpose. “
Caroline Curry, whose 19-year-old son Liam was killed in the attack, said: “From top to bottom, MI5 to the attacker’s associates, we will always believe that you all played a part in the murder of our children.”
Figen Murray, the mother of Martyn Hett, 29, who also died in the blast, said she accepted MI5’s apology although “it didn’t surprise me that there is another catalog of errors and omissions”.
“I’m not excusing anyone who makes a mistake, don’t get me wrong, but these people are human and I’m sure nobody made any mistakes on purpose.”
“It’s hard to hear that things could have been different, but you know what, we can’t turn back the clock,” she said, adding that it would be a “disservice” to her son’s “people-loving” and “fun-loving” self. “the nature “if I became a bitter and angry person”.
Richard Scorer, lead lawyer at Slater and Gordon, which represented 11 of the bereaved families at the inquest, said the report had been “deeply painful to read but also eye-opening” while providing “less information than we would have liked”.
Nicola Brook, of Broudie Jackson Canter, who is representing five bereaved families, said it was “disappointing that the families will never know the full truth about what happened”.
“All the families signed a pledge not to disclose confidential information that they have not breached,” she added.
“They, above all others, have a right to know what the security services knew and had the most interest in keeping it confidential.”
After publishing the report, MI5 said since the attack it had made more than 100 improvements.
“But we are determined to do more. As the chairman now considers his recommendations, we will be fully engaged,” McCallum said.
“Where there are opportunities to further strengthen the UK’s defences, MI5 will act.”
A spokesman for Prime Minister Rishi Sunak said the government would look at the report’s findings before responding fully and formally.
Asked if anyone would be held accountable, Downing Street said it could not anticipate a formal response, but added ministers “will of course learn from this to ensure mistakes are not repeated”.
Home Secretary Suella Braverman said she was “committed to working with MI5, the police and partners to study the recommendations”.
“Together we will do everything to prevent a repeat of this horrific attack,” she added.