The information work MI5 missed a important accidental to instrumentality enactment that mightiness person prevented the Manchester Arena attack, an enquiry has found.
Chairman Sir John Saunders said the quality could person led to termination bomber Salman Abedi being followed to a car wherever helium stored his explosives.
Twenty-two radical were killed and hundreds injured successful the 2017 bombing.
MI5 director-general Ken McCallum said helium was "profoundly sorry" the information work "did not forestall the attack".
"I profoundly regret that specified quality was not obtained," helium said.
"Gathering covert quality is hard - but had we managed to prehend the slim accidental we had, those impacted mightiness not person experienced specified appalling nonaccomplishment and trauma."
The enquiry recovered 2 pieces of accusation astir Abedi were assessed astatine the clip by the information work arsenic not being terrorism-related.
One serviceman admitted they considered a imaginable pressing nationalist information interest connected 1 of the pieces of quality but did not instantly sermon it with colleagues and did not constitute up a study that aforesaid day.
In his 207-page report, Sir John said: "The hold successful providing the study led to the missing of an accidental to instrumentality a perchance important investigative action.
"Based connected everything the information work knew oregon should person known, I americium satisfied that specified an investigative enactment would person been a proportionate and justified measurement to take.
"This should person happened."
But helium added that Abedi "demonstrated immoderate information consciousness and that this mightiness person affected the efficacy of the investigative enactment that I person identified".
Sir John said the quality could person led to Abedi being followed to the parked Nissan Micra wherever helium stored his explosives and aboriginal moved them to a city-centre rented level to assemble his bomb.
He said that if MI5 had acted connected the quality received past Abedi besides could person been stopped astatine Manchester Airport connected his instrumentality from Libya 4 days earlier the attack.
The nationalist enquiry besides recovered Abedi was astir apt assisted by idiosyncratic successful Libya but it was not possible, connected the disposable evidence, to accidental who this mightiness person been.
It is the archetypal clip an authoritative decision has been made astir the imaginable engagement of different radical from abroad.
In making this finding, Sir John contradicts an MI5 appraisal which said no-one different than Salman Abedi and his member Hashem were knowingly progressive successful the plot.
The study besides recovered that portion Didsbury Mosque successful southbound Manchester, wherever the Abedi household worshipped, was not an progressive origin successful the brothers' radicalisation, politicisation did hap there.
Sir John said "there was a signifier of wilful blindness" to immoderate activities, and "weak leadership".
The study concluded that the Abedi household held "significant responsibility" for the radicalisation of Salman and Hashem Abedi.
Those household members liable see their begetter Ramadan Abedi, parent Samia Tabbal and elder member Ismail Abedi, each of whom has held extremist views, the enquiry found.
But Sir John said, different than Hashem Abedi, determination was insufficient grounds to property circumstantial cognition of the onslaught to different household members.
Sir John's comments were published successful the inquiry's 3rd and last report into the atrocity, which dealt with the radicalisation of Abedi and whether the onslaught could person been prevented.
The archetypal measurement has been made publically disposable portion the 2nd has lone been circulated to a constricted readership of radical who person information clearance.
This does not see the families of those who died, their ineligible teams, oregon the media arsenic its contents would beryllium damaging to nationalist information if made public, the enquiry was told.
The enquiry process began much than 3 years ago, and determination were 194 days of oral grounds from 267 witnesses.
A minute's soundlessness was held astatine Manchester Town Hall for the 22 victims of the arena bombing earlier Sir John work retired his findings including cardinal recommendations.
It seems a contradiction of the word "public inquiry" to support immoderate findings private, but the president says that helium had to equilibrium the rule of unfastened justness with the contented of nationalist security.
He's added that each backstage grounds has been scrutinised to marque definite that restricting it can't beryllium seen arsenic a "cloak to screen up mistakes".
Given that the erstwhile 2 enquiry reports were highly captious of the backstage companies and nationalist authorities progressive with the arena and the exigency response, this papers was besides expected to beryllium pugnacious connected MI5.
Bereaved families and survivors volition beryllium pleased that successful immoderate areas the study doesn't propulsion its punches.
They'll find it upsetting to perceive that the information work missed a important opportunity. But determination volition besides beryllium immoderate vexation that they'll ne'er larn the afloat item of precisely what that accidental involved.
Andrew Roussos, whose eight-year-old girl Saffie-Rose was killed successful the blast, said proceeding however this calamity mightiness person been avoided was "devastating for america all".
"This was a cataclysmic failure, and it is wide from each of the grounds we person heard astir Abedi that determination were galore opportunities for the information services to person ensured the bombing ne'er happened.
"In my presumption the information that MI5 failed to halt him contempt each of the reddish flags disposable demonstrates they are not acceptable to support america harmless and truthful not acceptable for purpose."
Caroline Curry, whose lad Liam, 19, was killed successful the attack, was killed successful the attack, said;: "Forgiveness volition ne'er beryllium an enactment for specified evil intentions and those that played immoderate portion successful the execution of our children volition never, ever get forgiveness.
"From apical to bottom, MI5 to the associates of the attacker, we volition ever judge you each played a portion successful the execution of our children."
Richard Scorer, main lawyer astatine Slater and Gordon, who represented 11 of the bereaved families astatine the inquiry, said the study had been "deeply achy to read, but besides eye-opening" portion providing "less accusation than we would person wanted".
Nicola Brook, from Broudie Jackson Canter, which represents 5 bereaved families, said it was "disappointing that the families volition ne'er cognize the afloat information of what happened".
"All of the families signed an undertaking not to uncover confidential accusation which they person not breached.
"They, supra each others, are entitled to cognize what the information services knew and had the astir involvement successful keeping it confidential."
Following work of the report, MI5 said since the onslaught it had made much than 100 improvements.
"But we are determined to bash more. As the seat present considers his recommendations, we volition prosecute fully," said Mr McCallum.
"Where determination are opportunities to fortify the UK's defences further, MI5 volition act."
Home Secretary Suella Braverman said she was "committed to moving with MI5, policing and partners to survey the recommendations".
"Together we volition bash everything imaginable to forestall a repetition of this horrifying attack," she added.
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