Manchester attack: Bomber not acting alone, says Amber Rudd
The man who carried out a suicide attack in Manchester was "likely" to have not acted alone, 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 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 allow for the redeployment of police officers, the Ministry of Defence said.
Mrs Rudd said: "[Monday's attack] 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."
BBC security correspondent Frank Gardner said that the bomber is thought to have been a "mule", using a device built by someone else.
Who are the victims?
The victims include Nell Jones, 14, eight-year-old Saffie Roussos, Alison Howe, Lisa Lees, Jane Tweddle-Taylor, 50, Martyn Hett, 29,Olivia Campbell, 15,Kelly Brewster, 32, John Atkinson, 28, Georgina Callander - thought to be 18 - and Marcin and Angelika Klis, a Polish couple from York.
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.
Greater Manchester Police said it was "confident" that officers know the names of all those killed. It said that it had made contact with all of the families.
It would formally name the victims after the post mortems, a process likely to take four or five days.
A hotline has been set up for people concerned about loved ones - 0800 096 0095.
What does a 'critical' threat level mean?
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, Mrs 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".
Mrs Rudd said 984 troops had been deployed in the first instance. Up to 3,800 are available.
She said she "absolutely" expected the raising of the threat level to critical to be temporary, adding that the bomber had been known "up to a point" by the intelligence services.
Mrs 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.
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.
These include 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".
Who was the attacker?
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.
He attended Burnage Academy for Boys in Manchester between 2009-11.
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.
A former classmate of Abedi's has told the BBC that "he was a very jokey lad" but was at the same time was "very short tempered", becoming angry at "the littlest thing".
"He had a short temper but apart from that was a very sound lad," said the man, who did not want to be identified.
He said that Abedi was "away at random times throughout the year. but I don't know if that was because he was out the country or just didn't show up to school, because he did hang around with the wrong crowd and was very, very gullible."
"You could tell him anything and he would pretty much fall for it."
He said that, before leaving the school in 2011, Abedi became "more and more religious" and that this might explain why he cut ties with former classmates.
What's happening with the investigation?
Apart from the three arrests in south Manchester on Wednesday, Abedi's older brother Ismael 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.
Meanwhile, a man with a knife has been arrested near Buckingham Palace, but Scotland Yard said this incident was not believed to be terror related.
How has Manchester reacted?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.
Manchester metro mayor Andy Burnham told the BBC that the attack had been the city's "darkest hour but also you say the best of Greater Manchester".
He said: "I was in the hospitals late last night and I was hearing stories that porters, cleaners, surgeons, nurses, came in from not being on shift to help out. The public were bringing food. The people really did pull together and I think we should take a great deal of pride in that."
