"It is conceivable that the Manchester attack in particular might have been averted if the cards had fallen differently", Anderson stated.
On two occasions in the months before Abedi killed 22 people, MI5 received intelligence, but its significance was not fully appreciated at the time and, in hindsight, was "highly relevant" to the planned attack.
The report by David Anderson QC, Britain's former independent reviewer of terror legislation, reviews a series of terrorist attacks in the United Kingdom this year and stops short of apportioning blame on MI5 and counter terror police for failing to stop four attacks since March. Had an investigation been reopened at the time, it cannot be known whether Abedi's plans could have been stopped.
Salman Abedi, a 22-year-old Briton born to Libyan parents, blew himself up at the end of a show by U.S. singer Ariana Grande in the deadliest militant attack in Britain for 12 years.
The four attacks killed a total of 36 people and wounded 200 others.
"It is not the goal of the internal reviews, or of this report, to cast or apportion blame".
Parker said that intelligence regarding Abedi was thought to involve possible gang activity rather than terrorism, admitting that Abedi should have been stopped at the airport when he returned to the United Kingdom from Libya.
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An automated sweep of suspects' data created to identify closed cases that may need re-examining "identified Salman Abedi as one of a small number of individuals, out of a total of more than 20,000 closed subjects of interest, who merited further examination". The attack at Parsons Green, on 15 September, was not covered by the report.
Authorities failed to stop Abedi when he returned to the United Kingdom from Libya, according to the findings.
Abedi could have been stopped had MI5 interpreted two key bits of intelligence differently according to the findings of Ex-counter terror chief David Anderson.
He said the home secretary, Amber Rudd, told her colleagues that the pace of attack planning had increased significantly this year, and insisted that ministers were putting pressure on social media companies to remove terrorist material and progress was being made.
A review three of six attackers "were on MI5's radar".
Butt was the "principal subject" of an MI5 probe called Operation Hawthorn that "opened in mid-2015 following information suggesting that he aspired to conduct an attack on the United Kingdom", according to the Telegraph. It was scheduled for May 31, nine days after the attack.
"After four such incidents over a short period, unsparing reflection was required".