MANCHESTER, England (CNN) -- The 22-year-old behind the deadly bombing at an Ariana Grande concert in Manchester was known to security services and was linked to a wider network, British officials said Wednesday, as the country elevated its terror threat to the highest level for the first time in a decade.
Police have named Salman Abedi, a British-born national of Libyan descent, as the bomber in the attack on Manchester Arena, which killed at least 22 people, including children. Abedi died in the blast, in what appears to have been a suicide bombing.
"It is very clear that this is a network we are investigating," Greater Manchester Police Chief Constable Ian Hopkins said Wednesday following a series of raids and arrests after the attack.
UK Home Secretary Amber Rudd earlier told the BBC that Abedi was on the radar of intelligence services and that he had recently returned to the UK from Libya.
US military officials assigned to the Africa Command said that Abedi was in Libya for three weeks and returned to the UK only days before launching his attack. The officials said that information had been shared between US and British intelligence services, but cautioned that a full intelligence analysis was underway.
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Rudd earlier slammed the US for leaking information on the attack investigation as "irritating."
A string of details about the attack, including the bomber's name, have emerged from US law enforcement sources before being released by British officials.
French Interior Minister Gérard Collomb told BFM-TV that Abedi had "proven links to ISIS," without elaborating.
"The intelligence services know a lot of people, and I'm sure we will find out more what level they knew about (Abedi) in due course. But at the moment all they have confirmed is that they did know about him. And as I say, we will find out more when the operation is complete," Rudd said Wednesday.
In an interview with Sky News, Rudd refused to respond to Collomb's comments that Abedi was believed to have been to Syria. She also refused to elaborate on reports in Arab media that the father of the bomber had links to Islamist rebel groups in Libya.
Six of the 22 victims in the attack, including children, have been named, and Chief Constable Hopkins confirmed Wednesday that another person killed was a serving a police officer. He did not name that victim.
Security boosted
Prime Minister Theresa May announced Tuesday night that Britain's threat level had been raised from "severe" to "critical," and warned that a "further attack may be imminent."
Three men were arrested in south Manchester on Wednesday and two others were detained Tuesday.
CNN journalists witnessed the aftermath of a raid outside the Granby House apartment building in central Manchester on Wednesday, where police were closely guarding the front door and letting residents out. Dramatic images from the raid's start showed heavily armed officers storming the building.
Up to 3,800 military personnel have been made available across Britain following the attack, Secretary Rudd said, and almost 1,000 are now deployed.
London's Metropolitan Police service announced that military personnel would guard "key locations" as part of what's been called "Operation Temperer," and soldiers were seen at Buckingham Palace and extra police at train stations on Wednesday morning.
Police in London said a man with a knife was arrested near Buckingham Palace Wednesday morning, but they stressed that the incident was not terrorism-related.
"I would expect this to be temporary but we will keep a close eye to see how long we need them for and when it's appropriate we end Operation Temperer and go back to our different levels," Rudd said.
The Chelsea Football Club canceled its victory parade for winning the English Premier League. It was planned for Sunday in London, but the club said it decided to call it off out of respect for the victims and in light of the security threat.
The British parliament said that it was closing its doors to non-passholders and canceled all its events.
Attacker's ties to Libya
The suspected attacker has not yet been formally identified by the coroner, Manchester police said.
Abedi was of Libyan descent but born and raised in the UK, sources in Manchester's Libyan community told CNN. He was a student at the University of Salford in Manchester.
The University told CNN that he was studying business and management but while he was enrolled for the current academic year he has not been attending classes.
Questions have begun to swirl on Abedi's activities in Libya since it emerged that he recently visited the country.
Libya has become a hotspot for terrorism since the downfall and death of dictator Moammar Gadhafi in 2011. ISIS has gained a foothold in the country, and a tenuous UN-backed government there has struggled to assume full command of the security situation.
ISIS claimed Monday's attack, saying on its Telegram channel Tuesday that a "soldier of the caliphate" was able to "plant explosive devices" at the arena, according to a US counterterrorism source. ISIS routinely claims attacks it has no proven connection to.
Authorities have discovered no evidence of a link between the attacker and an established terror group, a British counter-terrorism official told CNN.
Children among the dead
Monday's blast marked the deadliest terror attack on British soil since the 2005 London bombings, which killed 52 people.
Six people who died in the horrific attack Monday have been identified -- they include an 8-year-old girl and two teenagers.
Concertgoer Olivia Campbell, 15, whose mother spoke to CNN during an agonizing wait for news from her daughter, had gone to the concert with her friend Adam to celebrate his birthday.
Eight-year-old Saffie Rose Roussos from Leyland was also named as one of the fatalities, Lancashire County Council confirmed.
Georgina Callander, 18, a superfan who had met Ariana Grande, was killed, according to her school, and John Atkinson, a 26-year-old student from the Greater Manchester area also died in the attack, according to Ivan Lewis, a local politician.
Two Poles were also named among the dead, according to a tweet from Poland's Foreign Ministry. No further details have been released.
At least 12 victims aged 16 or under were being treated at a children's hospital for serious injuries, some of them fighting for their lives, a Manchester health official said. Sixty-four people were injured in the attack.
CNN's Ray Sanchez, Paul Cruickshank, Samantha Beech, Darran Simon, Lauren Said-Moorhouse, Bryony Jones, Pamela Brown, Evan Perez and Ross Levitt contributed to this report.
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