The Manchester Arena bomber had been a “subject of interest” and opportunities to stop him were missed, a report says.
Its author, David Anderson QC, said it was conceivable the attack by Salman Abedi in May could have been avoided had “cards fallen differently”.
After the bombing, which left 22 people dead, and three terror attacks in London this year, counter-terror police and MI5 conducted internal reviews.
The Met Police said better resources and infrastructures were needed.
The major internal reviews, which remain largely secret, are summarised in Mr Anderson’s report, and show:
The Manchester bomber Salman Abedi had been a “Subject of Interest” for MI5. This means he was someone they were investigating between January and July 2014, and then again in October 2015.
On two occasions in the months before Abedi killed 22 people, MI5 received intelligence. However, its significance was not fully appreciated at the time and, in hindsight, was “highly relevant” to the planned attack
There was no security service port alert against Salman Abedi. He was not questioned at the border when he returned to the UK from Libya four days before the attack.
Two other attackers had been on MI5’s radar. Khuram Butt, the leader of the London Bridge and Borough Market attack, and Khalid Masood on Westminster Bridge.
The reviews showed that Butt had been identified as someone who wanted to attack the UK two years earlier.
He was still a “live subject of interest” who was under investigation at the time of the attack. However, it was more for his intention to travel to Syria and for radicalising others.
He was also the main target of “Operation Hawthorn” – but this was suspended twice because of a lack of resources after the Bataclan attack in France and the Westminster Bridge attack.