■ Prime Minister Theresa May announced on Tuesday night that Britain’s threat level had been raised to critical, its highest level, suggesting that another terrorist attack may be imminent.
■ The bomber who carried out the deadly attack at the Manchester Arena was identified by the police on Tuesday as Salman Abedi, 22, the son of Libyan immigrants.
■ Mr. Abedi died at the scene after detonating an improvised explosive device. The police were investigating whether he had any help.
■ Mr. Abedi had traveled to Libya multiple times, but the timing of his last trip is not known, according to a senior United States official, who spoke on the condition of anonymity.
■ The Islamic State claimed responsibility for the Monday night bombing that killed 22 people and wounded 59 others after a concert by the American pop star Ariana Grande.
■ Children and teenagers who were attending a concert were among the dead, the police said, including an 8-year-old girl. Ms. Grande was not hurt.
■ Part of the investigation led police officers in Manchester to arrest a 23-year-old man on Tuesday at a home southwest of the city center.
■ Britain’s leading political parties agreed to suspend campaigning ahead of the June 8 election out of respect for the victims of the attack.
View photographs from the aftermath of the attack.
What We Don’t Yet Know
The police have not provided any more details about the bomber, Mr. Abedi, nor have they commented on his motive. It is also not clear whether he received help.
The Islamic State claimed responsibility for the attack, identifying the bomber in its statement as one of the “soldiers of the caliphate,” but the extent of the militant group’s connection is unclear.
We are also trying to find out:
■ What planning went into the attack, and whether there are accomplices who are at large.
■ How many of the 22 victims were children.
■ What gaps in security at the arena might have abetted the attack, and what could have been done to prevent it.
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Raids and an Arrest in Manchester
The police have said they are following leads and poring over surveillance footage to determine if the bomber had help or was part of a wider network. Shortly before noon on Tuesday, the police announced that they had arrested a 23-year-old man in the Chorlton-cum-Hardy neighborhood, southwest of the city center. His connection to Mr. Abedi is not clear.
Hours later, the police set off a controlled explosion and raided a house in Elsmore Road, in the Fallowfield district, where Mr. Abedi lived with his family, according to neighbors. The house is about 3.5 miles south of the arena.
Read more about the investigation.
— Katrin Bennhold, Steven Erlanger and Ceylan Yeginsu
First Victims Are Identified
Saffie Rose Roussos, an 8-year-old from Lancashire in northwest England, was killed in the attack. She was attending the concert with her mother and older sister, the Lancashire County Council said on Tuesday. News reports said Saffie’s mother and her sister, who is in her 20s, were being treated at a hospital for their injuries.
Earlier, Georgina Bethany Callander, an 18-year-old health and social care student, was the first victim of the Manchester attack to be identified on Tuesday. News reports said she had died with her mother at her bedside.
Read more about the victims who have been identified here.
— Dan Bilefsky
Together, a Community Grieves
Thousands of people came together for a vigil in Manchester’s Albert Square on Tuesday evening to remember the victims and comfort one another.
Olivia Roberts, 15, who attended the concert but was not injured, was still wearing her Ariana Grande tour T-shirt that carried blood stains as she stood in the middle of the crowd and hugged her mother.
“I brought her here because she needs to process what has happened. She needs to grieve,” her mother, Lorraine, said as she stroked her daughter’s head with one hand and held up a bouquet of flowers with the other. “This is the world we live in now. We have to face up to it.”
After a minute’s silence in honor of the victims, the crowd broke out in various chants, “We love Manchester. Manchester will resist.”
Addressing the crowd, the Greater Manchester’s police chief, Ian Hopkins, said, “As your chief constable and as a father I cannot begin to imagine how anyone can carry out such an unthinkable act.”
Britain’s home secretary, Amber Rudd, and the leader of the Labour Party, Jeremy Corbyn, attended the event along with several faith leaders and Manchester residents from a range of backgrounds and ethnicities.
“This is about Manchester, our beloved city,” said Ian Cross, 64, a resident of the city for over 40 years. “Those young innocent girls were our children. We’re here for them,” he said as he broke down in tears, clutching tightly onto a bouquet of pink roses.
Azam Mirza, 24, a Muslim from Manchester, attended the vigil with a homemade placard that read, “Hate no one. Love everyone.”
“Journalists asked me if I was nervous to come out here today. Why?,” he asked. “I’m here to support my community and condemn these hateful acts, just like everybody else,” Mr. Mirza said.
Some Concertgoers Are Still Missing
Others in Manchester were still appealing for help finding loved ones who had attended the concert.
The soccer club Manchester City has opened part of its Etihad Stadium to be used as an emergency facility for families awaiting news of relatives still unaccounted for.
A number of local shops have sent food and supplies to the families waiting there.
— Dan Bilefsky, Philip Pan and Rory Smith
Theresa May Pledges British Way of Life Will ‘Prevail’
Speaking outside her Downing Street office on Tuesday, the British prime minister appealed for anyone with information about the bombing to come forward, and she vowed that Britain’s way of life would “always prevail.”
Mrs. May said the 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.”
Queen Elizabeth II offered her “deepest sympathy to all who have been affected by this dreadful event,” and Prince Charles and his eldest son, Prince William, also issued their own statements of condolence.
The British government did not make any immediate comment on the claim of responsibility by the Islamic State.
President Trump, speaking in Bethlehem in the West Bank, where he was on his first foreign trip as president, condemned the bombing as a “very horrible morning of death,” and pledged “absolute solidarity” with Britain.
— Sewell Chan and Prashant S. Rao
False Reports Quickly Circulate
Hours after the deadly attack in Manchester, false reports about the assailant and victims started to circulate across social media, often aided by Britain’s traditional news media.
Two of the country’s newspapers reported incorrectly that a gunman had been spotted near a hospital on the outskirts of the city in northern England. The local authorities quickly debunked that report through a tweet.
As in previous attacks across Europe, including those recently in France and Germany, false reports of people looking for victims of the attack also spread rapidly, racking up thousands of likes and retweets even though the information was incorrect.
Other posts included photographs of Ms. Grande, portraying her — falsely — in the aftermath of the blast.
— Mark Scott
The Terrorism Threat in Britain
The assault at the arena was the worst terrorist attack in Britain since the 2005 bombings of London’s buses and subway, which killed 52 people.
British authorities, who say they have foiled numerous terrorist plots, had for months maintained the nation’s threat level, set by the domestic intelligence service MI5, at “severe,” the second-highest level. That means the authorities considered an attack “highly likely.” And counterterrorism officials have been warning that as the Islamic State comes under more military pressure in Iraq and Syria, it will try to strike abroad.
— Steven Erlanger
An Unrelated Terrorism Arrest
Soon after Britain raised its threat level on Tuesday night, the police in London said in a statement that they had arrested a 37-year-old man on “suspicion of preparing for acts of terrorism.” The arrest was unrelated to the Manchester attack.
The man was arrested at Stansted Airport before he boarded a flight to Turkey and the arrest is linked to travel to Syria, the Metropolitan Police said. Officers were searching two residential addresses in north London, they added.
— Niraj Chokshi
Blast Came After ISIS Plea
Last week, the Islamic State released a 44-minute video featuring fighters of different nationalities enjoining their supporters back home to carry out acts of violence. Among them was a man identified as a British citizen, according to a translation of the video provided by the Middle East Media Research Institute, which tracks jihadist propaganda.
Besides the threat last week, the Islamic State has repeatedly targeted Britain in its propaganda, though with little visible effect until this year.
— Rukmini Callimachi
Who Is Ariana Grande?
The 23-year-old singer and actress began her career on the children’s television network Nickelodeon. She played the character Cat Valentine — whom she once described as “simple” — on the hit shows “Victorious” and “Sam & Cat.”
Ms. Grande has written in a Facebook post that playing Cat Valentine had helped her transition “from teenager to adult.”
The concert that was attacked on Monday was part of a tour to support Ms. Grande’s 2016 album, “Dangerous Woman.” Reviewing the tour’s February stop at Madison Square Garden, The New York Times music critic Jon Pareles called Ms. Grande’s performance “a show of confidence, prowess and aplomb.”
In a tweet after the concert, she expressed her sorrow for the victims.
— Mike Ives
Reaction From Other Music Stars
‘God Save the Queen’ Plays at Yankee Stadium
The New York Yankees held a moment of silence before the game against the Kansas City Royals on Tuesday night for the victims of the attack. It was followed by a rendition of Britain’s national anthem, “God Save the Queen.”
“We stand in solidarity and unity with the people of the United Kingdom of Great Britain,” said Paul Olden, the Yankees public-address announcer.
— Matthew Haag
Arena Largest Such Venue in Britain
The Manchester Arena is the largest indoor venue in Britain, with a capacity of 21,000, or 18,000 for concerts. It was constructed as part of Manchester’s bid to host the Olympic Games in 1996 and 2000. The venue is connected to the Manchester Victoria Station, which remained closed on Tuesday.
The Pennsylvania-based company SMG manages the arena, and Wes Westley, the president and chief executive of the company, described the precautions at the venue.
“It is obviously as tight security as anywhere in the states,” he said in an interview. “Backpacks are not allowed. Drinks are taken away from people. You have to go through very strict security to enter the arena.”
He explained that attendees arrive through a large public foyer, which is where the explosion occurred. The area is often where parents wait for their children after concerts.
— Ben Sisario and Gerry Mullany