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 — following 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."
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 was registered as having lived.
A senior European intelligence official said 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. "We have an ISIS problem in Libya. We wonder whether he met people there who trained him," said the friend, who spoke on the condition of anonymity. ISIS is another name for the Islamic State.
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.
"Over the coming days as you go to a music venue, go shopping, travel to work or head off to the fantastic sporting events, you will see more officers, including armed officers," said Commander Jane Connors of London's Metropolitan Police Department.
May's decision to deploy the military means the public may now see soldiers rather than police. May said the military would operate under police command.
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.
To roaring applause, Manchester Mayor Andy Burnham vowed that the city — which has seen hardship, having been bombed relentlessly during World War II — would not succumb to division or anger. A poet named Tony Walsh delivered an ode to the city titled "This Is the Place." And in what has become a dark mainstay of life in Western Europe, passersby left candles, flowers and cards for the dead.
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 Wednesday, with authorities bracing the public for deaths among the teens and tweens who form the core of Grande's enthusiastic fan base.
The Islamic State did not give any details about the attacker or how the blast was carried out, raising doubts about the truth of its claim. Its statement was posted on the online messaging service Telegram and later noted by the SITE Intelligence Group, which monitors militant websites.
The Islamic State often quickly proclaims links to attacks, but some previous boasts have not been proved.
In Washington, Director of National Intelligence Daniel Coats said Tuesday that despite the group's statement, "we have not verified yet the connection." He noted in a Senate hearing that "they claim responsibility for virtually every attack."
Wave of revulsion
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.
May later visited Manchester, meeting with local authorities and signing a condolence book honoring the victims.
Queen Elizabeth II, meanwhile, led guests of a garden party at Buckingham Palace in a moment of silence and issued a statement expressing her "deepest sympathies."
"The whole nation has been shocked by the death and injury in Manchester last night of so many people, adults and children, who had just been enjoying a concert," she said.
Across the world, other leaders expressed revulsion and scorn toward the bomber.
During a visit to the West Bank city of Bethlehem, President Trump pledged "absolute solidarity" with Britain and called those responsible for the attack "evil losers in life."
Organizers of the Cannes Film Festival denounced the bombing as an "attack on culture, youth and joyfulness" and observed a minute of silence Tuesday. Cannes is 15 miles from Nice, where an attacker driving a truck plowed into crowds celebrating Bastille Day in July, killing 86 people.
The Monday night attack was the worst terrorist strike on British soil since 2005, when Islamist extremists bombed the London subway and a bus, killing 54 people.
And as with that attack, Monday's bombing prompted desperate searches for missing loved ones that continued through the night and into Tuesday.
Charlotte Campbell told the BBC that she was "phoning everybody," including hospitals, trying to locate her 15-year-old daughter, Olivia. She last spoke to her daughter Monday night while she was at the concert.
"She'd just seen the support act and said she was having an amazing time, and thanking me for letting her go," Campbell said in an emotional interview.
The attack occurred near one of the exits of the arena, in a public space connected to a bustling train station.
Jake Taylor, a former security guard at the arena, said its layout makes absolute safety impossible.
"You can't stop people from getting through the train station," Taylor said.
Mark Harrison, who accompanied his 12-year-old daughter to the concert from Cumbria in northern England, said there were no metal detectors or body checks at the arena's entrance, although bags were inspected and items such as water bottles had to be discarded.
"There was definitely a security presence, but anyone can come through the train station," said Harrison, 44.
In France, the scene of several terrorist attacks in recent years, Prime Minister Édouard Philippe called on people to be vigilant in the face of "a threat which is more present than ever before."
Britain's threat level had been classified as "severe" since the summer of 2014, meaning the chance of an attack at any given time was highly likely.
Pantucci, the security expert, said that authorities had disrupted several plots in recent months but that Monday's attack somehow slipped through. Understanding why, he said, will be crucial.
"They've been dealing with a very high threat tempo," he said. "But this is one they weren't able to stop."
Adam reported from London. Isaac Stanley-Becker, James McAuley and Rick Noack in Manchester; Paul Schemm in Addis Ababa, Ethiopia; and Devlin Barrett, Brian Murphy and Ellen Nakashima in Washington contributed to this report.