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 redeploy police officers, the Ministry of Defence said.
Mrs Rudd said: “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.”
BBC security correspondent Frank Gardner said that the bomber is thought to have been a “mule”, using a device built by someone else.
Eight of the victims are known to be eight-year-old Saffie Roussos, Lisa Lees, Jane Tweddle-Taylor, 50, Martyn Hett, 29, Olivia Campbell, 15, Kelly Brewster, 32, John Atkinson, 28, and Georgina Callander – thought to be 18.
Two Polish people, Marcin and Angelika Klis, were also among those killed killed. They had been living in York for some time.
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”.
A hotline has been set up for people concerned about loved ones – 0800 096 0095.
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, 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.
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.”
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.
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.
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.