The bomber behind the terror attack in Manchester that killed 22 people had been a "subject of interest" for security services and opportunities to stop him were missed, according to a review published on Tuesday.
Salman Abedi had been a subject of interest in 2014 and 2015 but was not under active investigation when he detonated his device at Manchester Arena in May outside a concert by US pop star Ariana Grande.
The MI5 domestic security service had, however, come across intelligence in the months before the attack which, "had its true significance been properly understood", would have caused a new investigation into him to be opened.
The report said Abedi was also identified in a "data-washing exercise" as requiring further investigation but a meeting to discuss how to proceed had not been held when the attack happened.
Because Abedi was not on any security service watchlist he was not investigated when he returned to Britain from Libya just a few days before the attack.
"It is conceivable that the Manchester attack... might have been averted had the cards fallen differently," the report concluded.
The findings were in major reviews, which were launched after a series of attacks in Britain this year and remain largely secret, and were summarised in a report by senior lawyer David Anderson.
Home Secretary Amber Rudd, presenting the reviews in parliament, warned against pinning the responsibility on the work of the security services.
"We should be united in our clarity that it lies squarely with those whose cowardly acts killed 36 innocent people this year, and those who encouraged them," she told parliament.
- 'Jihad document' -
The report also found that Khalid Masood, who carried out an attack outside the British parliament in March in which five people were killed, was known to police and MI5 for associating with extremists.
But the report found that Masood was a "closed" subject of interest at the time of the attack in which he mowed down pedestrians in his car on Westminster Bridge and stabbed to death a police officer guarding the British parliament.
It also said that Masood had scoped out the area in the days before the attack both in person and online and browsed for terrorism videos on YouTube.
He also shared a "jihad document" with numerous WhatsApp contacts before launching the attack.
The report found that Khuram Butt, who led a three-strong attack team that killed eight people in and around London Bridge in June, had been the main target of an MI5 investigation from 2015.
But the investigation, known as "Operation Hawthorn", was suspended twice due to a lack of resources and had only resumed on the day of the attack.
Another of the London Bridge attackers, Youssef Zaghba, an Italian-Moroccan, had previously tried to travel to Syria and was put on an EU warning list in March.
But the reviews found Italian authorities had placed him on a database as being subject to checks for serious crime rather than a national security risk.
MI5 had received an inquiry from Italian authorities about Zaghba but had no record of responding "noting by way of possible explanation that it arrived in the incorrect mailbox".
The classified reviews contain 126 recommendations for the security services, including using data more effectively, sharing knowledge more widely and improving their own internal collaboration.