The Manchester bombing could have been stopped "had the cards fallen differently", an official assessment has found.
It discloses that three terrorists involved in four attacks that hit Britain between March and June this year had at some point been on authorities ' radar.
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".
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."
Salman Abedi was not under active investigation when he detonated a suicide device at 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: "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.
Police and MI5 have foiled nine plots since the Westminster attack in March, in addition to the terrorist incidents that have occurred.
Outlining the review findings, Home Secretary Amber Rudd told MPs that blame for the "cowardly" attacks "lies squarely" with the terrorists and those who encouraged them.
Greater Manchester Police Chief Constable Ian Hopkins said:
The scale of the current threat is seen as unprecedented.
Counter-terror teams are running about 500 live investigations involving 3,000 individuals at any one time, while there is also a wider pool of 20,000 subjects previously looked at who must be kept under review.
Mr Anderson, a former anti-terror laws watchdog, said:
The full internal reviews, which are highly classified and run to more than 1,100 pages, contain 126 recommendations.