The suicide bomber who killed 22 people after an Ariana Grande concert was part of a network and not a lone-wolf attacker, police said Wednesday as troops were deployed near landmarks and three more suspects arrested.
"It is very clear this is a network we are investigating," Manchester police chief Ian Hopkins said. "We are carrying out extensive searches."
Earlier, interior minister Amber Rudd told the BBC that suspected bomber Salman Abedi "was known" to intelligence services. She added that up to 3,800 soldiers would be put on Britain's streets.
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The presence of heavily armed troops in urban areas is highly unusual in the U.K. More than 90 percent of police officers tasked with protecting London's 8.5 million residents do not carry guns.
Three men were also detained in south Manchester in connection with the case — bringing the total in custody to four. On Wednesday afternoon, police said they were searching a building in the city's center after a controlled explosion to force their way inside.
The United Kingdom raised its terrorist threat level to "critical" — the highest category — in the wake of Monday's attack.
Troops were sent to key locations in London, such as Buckingham Palace, the prime minister's residence at 10 Downing Street, Parliament and foreign embassies.
Earlier, a U.S. intelligence official who has direct knowledge of the investigation had told NBC News that Abedi's device was "big and sophisticated," using materials hard to find in Britain — meaning "it's almost impossible to see he didn't have help."
Abedi — a 22-year-old British national whose family is of Libyan descent — had ties to al Qaeda, received terrorist training abroad and traveled to Libya within the last 12 months, the source added.
A "follow-on" attack is possible, the official said.
France's interior minister said Wednesday that Abedi was believed to have traveled to Syria and had "proven" links to ISIS. He did not provide details.
ISIS claimed responsibility for the attack on Tuesday but did not provide any evidence.
Manchester police said that they were now confident they knew the identities of all those who had died in the bombing. One of them was a police officer, according to Hopkins.
Separately, London police said they had arrested a man with a knife near Buckingham Palace, adding in a statement that the incident was not thought to be "terrorist related."