MI5’s internal investigation concluded that the decision not to reopen an investigation into Abedi in early 2017 was “finely balanced” and “understandable”.Reviewers decided that ”on the clear balance of professional opinion, a successful pre-emption of the gathering plot would have been unlikely”.Mr Anderson’s review three of the six attackers who conducted “were on MI5’s radar”. As well as Abedi, Khurram Butt, one of the London Bridge attackers, was the subject of active investigation, while Khalid Massood, the Westminister Bridge attacker, was also a former subject of interest. Butt was the “principal subject” of an MI5 investigation called Operation Hawthorn “opened in mid-2015 following information suggesting that he aspired to conduct an attack on the UK”.Under Hawthorn, he was given a priority ranking “signifying high extremist activity linked to attack planning”.He was subject to extensive surveillance, but as time passed, his focus was judged to have moved away from attacking the UK and towards travelling overseas to fight for Islamic State group.It was also disclosed that Khalid Masood, the Westminster attacker, had watched suicide attack videos on YouTube in the days before he drove at pedestrians and stabbed to death a policeman during an 82 second rampage. It found the 22-year-old who was born in Manchester to parents who had fled Libya had first become an MI5 “subject of interest” in 2014, but it transpired he had been mistaken for someone else and his case was closed. It was reopened the following year on mistaken intelligence that he had contacted an Islamic State figure in Libya.But though his case remained closed from that point, “Salman Abedi continued to be referenced from time to time in intelligence gathered for other purposes.”In two separate instances in the run up to the attack, intelligence was received that was “assessed at the time to relate not to terrorism, but to possible non-nefarious activity or to criminality”.An automated trawl of suspects data designed to spot closed cases that could 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”.“A meeting (arranged before the attack) was due to take place on May 31: Salman Abedi’s case would have been considered, together with the others identified. The attack intervened on May 22.”Mr Anderson said: “With the benefit of hindsight, intelligence was misinterpreted in early 2017.” Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Between March and June, London and Manchester saw four attacks killing a total of 36 people and wounding another 200. MI5 had planned to discuss the threat posed by the Manchester suicide bomber just nine days after he launched his attack, an official report has revealed.The review by David Anderson QC found the bombing which killed 22 people could have been averted “had the cards fallen differently.”Salman Abedi had been highlighted for discussion at an MI5 meeting due to take place on May 31, but he detonated his explosives at the Manchester Arena nine days earlier.Internal reviews into the police and MI5’s handling of the four terrorist attacks to strike Britain this year said MI5 had on two separate occasions received intelligence on Abedi “whose significance was not fully appreciated at the time”.The report by David Anderson QC, a former terrorism law reviewer asked by the Home Secretary to audit internal MI5 and police reviews, said: “In retrospect, the intelligence can be seen to have been highly relevant to the planned attack.”His 61-page report concluded it was “conceivable that the Manchester attack in particular might have been averted had the cards fallen differently”.Rudd: Nine plots foiled since Westminster attack Mr Anderson said: “The excellent recent record of MI5 and police in defending the UK from terrorist attack came to a brutal end this year at Westminster, Manchester Arena, London Bridge and Finsbury Park.”He said: “After four such incidents over a short period, unsparing reflection was required.”British spies have foiled nine terror attacks in the past 12 months, Andrew Parker, the head of MI5, told the Cabinet on Tuesday morning.Mr Parker disclosed the figures as part of a briefing on the current terrorist threat facing the UK.