British intelligence agency, MI5, was "actively" investigating the ringleader of the London Bridge atrocity at the time of the rampage, while the Manchester bombing could have been stopped "had the cards fallen differently".
On Tuesday an official review by David Anderson QC found three terrorists involved in four attacks that hit Britain between March and June this year had at some point been on authorities ' radar.
UK authorities were questioned after dozens of victims at Westminster, Manchester, London Bridge, and Finsbury Park.
"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."
Bomber Salman Abedi was not under active investigation when he detonated a suicide device at Manchester arena in May, which killed 22 people.
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Anderson's review found 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.
Westminster attacker Khalid Masood was known to police and MI5 for association with extremists, but authorities had no reason to anticipate his murderous actions.
Khuram Butt, who led the three-strong gang behind the London Bridge van and knife attack in June, which left two young Australian women dead, was the principal subject of an MI5 investigation from mid-2015 until 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 was deleted by Italian authorities in January.
Zaghba, and the third London Bridge attacker Rachid Redouane, were never investigated by MI5.
MI5 and police have together thwarted 22 plots in the last four years, nine of which have been stopped since March 2017, the police said.
