British Prime Minister Theresa May on Tuesday night raised the nation's threat level and deployed the military to guard concerts, sports matches and other public events, saying another attack "may be imminent" after a bombing Monday night that left 22 people dead.
The announcement, which takes Britain's alert level from "severe" to its highest rating, "critical," clears the way for thousands of British troops to take to the streets and replace police officers in guarding key sites.
May announced the move after chairing an emergency meeting of her security Cabinet and concluding that the attacker who carried out Monday's bombing may have been part of a wider network that is poised to strike again. The decision, she said, was "a proportionate and sensible response to the threat that our security experts judge we face."
The worst terrorist attack on British soil in over a decade was carried out by a 22-year-old British citizen who lived a short drive from the concert hall that he transformed from a scene of youthful merriment into a tableau of horror.
But whether Salman Abedi acted alone or with accomplices remained a question that British investigators were urgently trying to answer Tuesday night as they reckoned with an attack more sophisticated and worrisome than any seen here in years.
The prospect of a wider plot, May said, was "a possibility we cannot ignore."
The killing of 22 people — many of them teens — after a concert in this northern English city by American pop star Ariana Grande was claimed Tuesday by the Islamic State, which said one of its "soldiers" was responsible.
Even as officials and experts cast doubt on the terrorist group's assertion, however, authorities were scrambling to execute searches, arrest potential accomplices and reinforce security systems at a spectrum of public events that look newly vulnerable to attacks like Monday's.
After years of successfully fending off more-sophisticated strikes even as countries across continental Europe have fallen victim to bombings, Monday night's carnage underscored that Britain is not immune amid a rising tide of extremist violence.
The highest priority for police, said Greater Manchester Chief Constable Ian Hopkins, was to "establish whether Abedi was acting alone or as part of a network."
Earlier he had said that Abedi executed the bombing alone and that he "was carrying an improvised explosive device, which he detonated, causing this atrocity."
But unlike in previous high-profile attacks — including one in March in which an assailant driving a speeding car ran down pedestrians on a London bridge, then stabbed to death a British police officer — experts said it was unlikely that Monday's attack had been carried out without help.
"Getting a car or a knife is easy," said Raffaello Pantucci, a terrorism expert at the London-based Royal United Services Institute. "Making a bomb that works and goes off when you want it to go off takes preparation and practice. And it usually involves other people."
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Pantucci said British authorities "are going to try to figure out who Abedi knows, who he's linked to. Did he build the bomb itself, or did someone build it and give it to him?"
If police have an answer, they did not say so publicly Tuesday. But there was ample evidence of a widening security operation, with the arrest of a 23-year-old from south Manchester in connection with the bombing. Police also carried out searches at two homes, including the house in the leafy suburban neighborhood where Abedi's was registered as having lived.
A senior European intelligence official told the Washington Post that the attacker was a British citizen of Libyan descent. The official, who was not authorized to speak on the record and thus spoke on the condition of anonymity, said the suspect's brother has been taken into custody.
A family friend said Abedi traveled frequently between Libya and Britain. The friend, who spoke on the condition of anonymity, wondered whether Abedi met people in Libya who trained him.
Even before May's announcement of a "critical" threat level for just the third time ever — the first two came in 2006 and 2007 — authorities from London to Scotland said that they would be reviewing security plans for upcoming public events. Even smaller gatherings that would not have been policed in the past may now get protection, they said.
The escalation came as the nation grieved for the young victims, with thousands of people converging on Manchester's graceful Albert Square for a vigil that was part solemn remembrance and part rally against extremism.
The casualties included children as young as elementary school students. Police said that among the 59 people injured, a dozen were younger than 16.
Among the dead was Saffie Rose Roussos, who was just 8 years old. The first victim to be publicly identified was Georgina Callander, an 18-year-old student.
Other names were expected to be released today, with authorities bracing the public for deaths among the teens and tweens who form the core of Grande's enthusiastic fan base.
In a speech outside 10 Downing Street, where flags were lowered to half-staff, May called the Manchester killings a "callous terrorist attack."
"This attack stands out for its appalling, sickening cowardice deliberately targeting innocent, defenseless children and young people who should have been enjoying one of the most memorable nights of their lives," she said.
President Donald Trump addressed the attack during a meeting in Bethlehem, saying that the attack preyed on "innocent children." He says this "wicked ideology must be obliterated. And I mean completely obliterated."
He called those responsible for the attack "evil losers in life."
Information from the Associated Press was used in this report.