The Manchester Arena bombing could have been prevented "had the cards fallen differently", an official assessment has found.
Salman Abedi was not under active investigation when he detonated a suicide device killing four Lancashire concert-goers along with 18 others, at the Manchester arena in May.
But Mr Anderson's review says that MI5 came by unspecified intelligence in the months before the attack which, "had its true significance been properly understood", would have caused an investigation into him to be opened.
The report says: "Substantial and appropriate coverage was in place around key individuals and mechanisms designed to assess risk were working as intended.
"MI5 and counter-terrorism policing got a great deal right: particularly in the case of Manchester, they could have succeeded had the cards fallen differently."
The report adds: "It is unknowable whether such an investigation would have allowed Abedi's plans to be pre-empted and thwarted. MI5 assesses that it would not."
Abedi was also identified by a separate "data-washing exercise" as falling within a small number of former subjects of interest who merited further consideration.
However, a meeting scheduled to consider the results of this process had not been held at the time of the bombing, in which 22 people were killed.
An opportunity was also missed to place Abedi on "ports action" after he travelled to Libya in April.
Chief Constable for Greater Manchester Police said that the police remained committed to bringing anyone involved in the attack to justice.
Chief Constable Ian Hopkins said: “This year 22 people in Manchester were murdered. Twenty two families were left devastated and there are hundreds of people suffering from physical or emotional trauma. None of us will ever forget that most awful of days.
“Our thoughts remain with all those affected and we remain committed to bringing anyone involved in this attack to justice.
“We welcome the report by Mr David Anderson QC, which provided independent assurance of the reviews undertaken by National Counter Terrorism Policing and MI5 in the aftermath of the terrorist attacks in London and Manchester.
“Counter Terrorism Policing in the UK is recognised internationally for its successes and strong partnership approach to defeating terrorists, because of that we will never stop learning or adapting to ensure that the response meets the changing threat.
“The size and scale of the threat from terrorism has been made so tragically clear this year. Greater Manchester Police will support Counter Terrorism Policing and the UK intelligence community in its response to this step change in threat and in adopting the recommendations in the Review.
“Further independent scrutiny will follow including inquests into the deaths of those who lost their lives. Greater Manchester Police will support those inquiries and with our partner agencies will continue to support those affected.”
The UK's security apparatus faced questions after dozens of victims were killed or injured at Westminster, Manchester, London Bridge, and Finsbury Park.
MI5 and police launched independent reviews to examine what was known about the terrorists before they struck, decisions made on intelligence and possible areas for improvement.
An independent assessment of the findings by David Anderson QC, concludes that there is "no cause for despair", saying most attacks continue to be successfully disrupted.
But he notes that, other than the case of Finsbury Park, it cannot be said that MI5 and police were "entirely blindsided".
Westminster attacker Khalid Masood was known to police and MI5 for association with extremists.
But he was a closed subject of interest at the time of the atrocity in March, and intelligence officers and police had no reason to anticipate his murderous actions, according to the report.
It also reveals how in the days prior to the attack, Masood conducted reconnaissance of Westminster Bridge in person and online, and browsed YouTube for videos relating to terrorism.
Minutes before he struck, the terrorist shared a "Jihad document" with numerous WhatsApp contacts.
Khuram Butt, who led the three-strong gang behind the London Bridge van and knife attack in June, was the principal subject of an MI5 investigation from mid-2015 until the date of the deadly assault.
The report says material relating to Butt received in the two weeks prior to the attack added little to the intelligence picture and did not identify activity that led up to the attack.
Another of the London Bridge gang, Youssef Zaghba, was placed on an EU warning list in March last year but a marker which would have automatically identified him as a national security risk was deleted by Italian authorities in January.
In June 2016, MI5 received an inquiry from Italian authorities about Zaghba but the agency has no record of responding - "noting by way of possible explanation that it arrived in the incorrect mailbox".
The request was not chased up by Italian officials.
Zaghba, and the third London Bridge attacker Rachid Redouane, were never investigated by MI5.
Police and MI5 have foiled nine plots since the Westminster attack in March, in addition to the terrorist incidents that have occurred.