MI5 is to hold an inquiry into the way it dealt with warnings from the public that the Manchester suicide bomber, Salman Abedi, was a potential threat.
Whitehall officials have acknowledged the security service was examining what assumptions had been made about the 22-year-old before last Monday's attack.
It later emerged it was alerted to his extremist views on three occasions.
Fourteen locations are being searched by police and 13 men remain in custody on suspicion of terror offences.
The latest person to be arrested was a 19-year-old man in the Gorton area of Manchester on Sunday. A 25-year-old man was held earlier in the Old Trafford area of the city.
MI5 has launched a "post incident investigation" into how the Manchester bomber was overlooked, while a separate report is being prepared for ministers and those who oversee the work of the service.
A Whitehall official said previously that Abedi was one of a "pool" of former subjects of interest whose risk remained "subject to review" by the security service and its partners.
Born in Manchester to Libyan parents aged 16, BBC Newsnight reported he fought against the Colonel Gaddafi regime with his father during the school holidays.
It was while at Manchester College that two people who knew Abedi have confirmed they made separate calls to an anti-terrorism hotline to warn the police about his extremist views.
Seven children were among those who died when the Abedi detonated a bomb on 22 May, at the end of a concert by US singer Ariana Grande at Manchester Arena.
Greater Manchester Police have made an appeal to the public for information about his movements since 18 May, when he returned to the UK from abroad.
His city centre flat was one of the last places he went - and where he may have made the final touches to his explosive device - before going to the arena, police have said.
Police said their investigation is "making good progress" and has around 1,000 people working on it.
In total 15 people have been arrested in Greater Manchester, Warwickshire and Merseyside - but a woman and a 16-year-old boy were later released without charge.
The UK threat level has been reduced from critical to severe and soldiers deployed to support the police will be gradually withdrawn from Monday night.
Additional security measures were in place for the Great Manchester Run on Sunday, which attracted tens of thousands of participants.
A huge round of applause for the emergency services followed a minute's silence for the victims and casualties.
There was also higher security at weekend events such as the FA Cup final in London.
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