Evidence heard at the Manchester Arena public inquiry has shown humanity both at its worst and at its best, the chairman said as he addressed families to mark the publication of his third and final report.
Sir John Saunders spoke movingly of an 'appalling loss which was so unnecessary' as he paid tribute to the 22 lives lost.
And he said that he 'can only hope that we achieve something by our efforts' - as he confirmed he found a 'significant missed opportunity to take action that might have prevented the attack'.
READ MORE: MI5 'missed significant opportunity' to stop Arena bomber - latest updates
The security service MI5, he said, didn't act swiftly enough over two crucial pieces of intelligence that were not passed on to police.
"It is not possible to reach any conclusion on the balance of probabilities or to any other evidential standard as to whether the attack would have been prevented," he said.
"However, there was a realistic possibility that actionable intelligence could have been obtained which might have led to actions preventing the attack.
"The reasons for this missed opportunity included a failure by the security service, in my view, to act swiftly enough."
Sir John said he was ‘satisfied’ the Abedi brothers used an instruction video placed on the Internet by Islamic State to create the bomb.
And he said more needs to be done to ensure ‘instructions for murder’ are not put up online.
The evidence as a whole, said Sir John, had often been 'gruelling and distressing to listen to'.
"For example, the evidence of people who were there and did their best to help, who will be affected for the rest of their lives by what they saw and heard; the evidence of the bereaved families of the effect that the deaths of their loved ones has had on their lives and who could fail to be moved by the tributes of the families to those who died.
"Those who were young were so full of promise; those who were older were achieving that promise and all of them were the cornerstones of their families. What an appalling loss which was so unnecessary.
"Amongst all that grief we should try and remember the positives as well along with the terrible events. The courage of members of the public and many members of the rescue services who did their best to help in appalling circumstances."
Sir John praised the response of Manchester to the attack 'to support all those who had been affected'.
"I am especially grateful to the bereaved families for their continuous support throughout the inquiry. It is impossible to imagine how difficult it must have been for them to listen to some of the evidence that we have heard.
"I have not been able to answer all of the questions that they wanted answered and I am aware that in my open report on volume three, I have raised more questions in their mind. I am sorry but that was inevitable. I did ask the questions; I did get answers; but for the reasons I have given I have not been able to report publicly what those answers were."
Sir John said violent Islamist extremists 'are a tiny group who have adopted a distorted view of Islam based on an incorrect interpretation of the Qur’an'.
And he said he has concluded there were 'a number of contributory factors to Salman Abedi’s radicalisation', including his parents and violent extremists the Abedi brothers met in Libya, which Sir John said were likely to include members of the Islamic State 'who would be in a position to provide the brothers with expertise in the making of bombs and in carrying out counter surveillance measures'.
"As leading counsel for the inquiry said during one of the hearings: how was someone of Salman Abedi’s limited intelligence and abilities able to succeed in committing this atrocity despite any safeguards which were in place.
"The likely answer is because he was provided with help."
Sir John said for him, it had been an 'enormous responsibility as well as a privilege' to conduct the inquiry.
"I can only hope that we achieve something by our efforts," he said.
"That will only happen if those away from this inquiry can share in the desire of those who have taken part in it to make things better.
"Inevitably some of the changes that are needed will require money which is in short supply but protecting the lives of the people of this country must be a high priority for any Government."
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