The man who carried out a suicide attack in Manchester was ‘likely’ to have not acted alone, UK home secretary Amber Rudd says.
Salman Abedi killed 22 and injured 64 when he blew himself up at the Manchester Arena on Monday night
Police arrested three men in Manchester on Wednesday. Abedi's 23-year-old brother was arrested on Tuesday.
The UK terror threat level is now up to its highest level of ‘critical’, meaning more attacks may be imminent.
It means military personnel are now being deployed to protect key sites.
The Palace of Westminster has been closed to the public following police advice, and will not re-open until further notice, a statement on its website said.
And the Changing the Guard ceremony at Buckingham Palace has been cancelled on Wednesday to redeploy police officers, the ministry of defence said.
Rudd said the attack was a ‘devastating occasion’.
‘It was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own,’ she added.
Six of the victims are known to be eight-year-old Saffie Roussos, Martyn Hett, Olivia Campbell, 15, Kelly Brewster, 32, John Atkinson, 28, and Georgina Callander - thought to be 18.
Two Polish people are also among those killed, according to a Polish government minister.
The injured are being treated at eight Greater Manchester hospitals. Of those, 20 are in a critical condition, and some have lost limbs.
The wounded include 12 children aged under 16.
Several people are still missing, including Eilidh MacLeod, 14, from Barra in the Outer Hebrides, Chloe Rutherford, 17, and Liam Curry, 19.
Eilidh's friend, Laura MacIntyre, 15 - who was also reported as missing - was later identified as one of the seriously injured in a Manchester hospital.
Manchester metro mayor Andy Burnham told the BBC that the attack had been the city's ‘darkest hour’.
Prime minister Theresa May said soldiers would be placed in key public locations to support armed police in protecting the public. These include Buckingham Palace, Downing Street, embassies and the Palace of Westminster.
Military personnel may also be seen at other events over the coming weeks, such as concerts, May said, working under the command of police officers.
The prime minister said she did not want the public to feel ‘unduly alarmed’ but said it was a ‘proportionate and sensible response’.
Rudd has said she ‘absolutely’ expects the raising of the threat level to critical to be temporary.
She also said the bomber was known ‘up to a point’ by the intelligence services.
Rudd also said there would be an ‘uplift’ in Prevent, the government's anti-radicalisation programme, after June. This had already been planned before Monday's attack, she added.
It is understood that between 400 and 800 troops will be deployed in the first instance. Up to 3,800 are available.
The highest threat level, which is decided by the Joint Terrorism Analysis Centre - a group of experts from the police, government departments and agencies - has only been reached twice before.
The first time the threat level was raised to critical was in 2006 during a major operation to stop a plot to blow up transatlantic airliners with liquid bombs.
The following year, security chiefs raised it once more as they hunted for the men who had tried to bomb a London nightclub, before going on to attack Glasgow Airport.
The metropolitan police says it has increased its presence across London, including specialist police officers who are trained ‘to spot the tell-tale signs that a person may be carrying out hostile reconnaissance or committing other crime... based on extensive research into the psychology of criminals and what undermines their activities’.
The change in terror threat comes after investigators were unable to rule out whether the bomber, named by police as Salman Abedi, had help carrying out the attack.
He is understood to be a 22-year-old born in Manchester to parents of Libyan descent, and a former Salford University student.
Hamid El-Sayed, who worked for the UN on tackling radicalisation and who now works at Manchester University, said Abedi had a ‘really bad relationship’ with his family.
He said, according to a family friend, that Abedi's parents had tried to ‘bring him back on the right path and they failed to do that’.
‘Eventually he was doing very bad at his university, at his education, and he didn't complete, and they tried to take him back to Libya several times. He had difficulties adjusting to European lifestyle.’
Abedi blew himself up in Manchester Arena's foyer shortly after 22:30 BST on Monday.
Fans were beginning to leave a concert by US singer Ariana Grande.
Witnesses at the arena described seeing metal nuts and bolts among the debris of Monday's bomb, and spoke about the fear and confusion that gripped concert-goers.
The arena bombing is the worst attack in the UK since the 7 July bombings in 2005, in which 52 people were killed by four suicide bombers.
So-called Islamic State has said - via IS channels on the messaging app Telegram - it was behind the attack, but this has not been verified.
Apart from the three arrests in south Manchester on Wednesday, Abedi's older brother was arrested in Chorlton, south Manchester, in connection with the attack.
Met Police Assistant Commissioner Mark Rowley, who is the national counter-terrorism policing lead, said the investigation was ‘fast-moving and making good progress’.
‘However, a critical line of inquiry is whether the dead terrorist was acting alone or part of a group,’ he said.
‘We still have critical lines of inquiry they're chasing down which has led to a level of uncertainty.’
Anyone with information about the attack can call the anti-terror hotline on 0800 789321.
Thousands of people turned out for the vigil in Manchester and to hold a minute's silence to remember those who died. Vigils were also held elsewhere.
Home secretary Amber Rudd, Labour leader Jeremy Corbyn and Commons speaker John Bercow stood on stage alongside Manchester mayor Andy Burnham and Greater Manchester Police chief constable Ian Hopkins.
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