MANCHESTER, England — Britain’s prime minister put the nation on its highest level of alert on Tuesday and deployed the military to work with the police over fears that another terrorist attack was imminent.
The announcement came as the police continued to investigate whether the Monday night bombing at a pop music concert in Manchester that killed 22 people, including children, was part of a broader conspiracy.
“It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack,” Prime Minister Theresa May said in Manchester after a meeting of her top security officials.
Earlier in the day, the police raided the home of Salman Abedi, the man they identified as the bomber; he died in the blast. Chief Constable Ian Hopkins of the Greater Manchester Police said that the investigation was focusing on determining “whether Mr. Abedi was acting alone or as part of a network.”
A senior United States official said on Tuesday night that Mr. Abedi had traveled multiple times to Libya, where his parents immigrated from, but did not know the timing of his last trip. The official was not authorized to discuss the information publicly and spoke on the condition of anonymity.
By raising the national threat level from severe to critical, Ms. May suggested “not only that an attack remains highly likely, but that a further attack may be imminent.”
The government’s actions on Tuesday night came hours after the authorities began the gruesome task of identifying the dead. An 8-year-old girl who had attended the Ariana Grande concert with her mother and older sister and a college student who chronicled on Instagram her encounters with her pop-music idols like Ms. Grande were among those killed.
As the authorities bolstered the nation’s defenses, investigators set out to learn as much as they could about Mr. Abedi, 22, who lived with his family only a few miles from where he detonated a homemade bomb on a public concourse crowded with Ms. Grande’s adoring teenage fans leaving the arena.
Rescue workers sifting through the carnage outside the arena on Monday night discovered Mr. Abedi’s identification card. That clue led the police to the home he shared with his family on Elsmore Road, in the Fallowfield district. The police blew the house’s door off its frame, to safeguard against booby traps, as shocked neighbors watched.
“We’ve been watching this kind of attack happen in Paris,” said a neighbor, Thomas Coull, 17. “We didn’t expect it to happen on our doorstep, literally.”
Mr. Abedi was born in 1994 in Britain, according to a law enforcement official speaking on the condition of anonymity because the investigation was still underway.
The Islamic State claimed responsibility for the attack, saying in one post on social media that “one of the soldiers of the caliphate was able to place an explosive device within a gathering of the crusaders in the city of Manchester.” It was one of several Islamic State statements, some contradictory, posted on different social media accounts.
A neighbor of the Abedi family in the Fallowfield district, southwest of the Manchester city center, said the family “didn’t really speak to anyone.” The neighbor, Lina Ahmed, added, “They were nice people if you walked past.” She said the family occasionally displayed a Libyan flag outside the home.
Another neighbor, Farzana Kosur, said that the mother, who taught the Quran, had been abroad for about two months. A trustee of the Manchester Islamic Center said Mr. Abedi’s father and his brother Ismael attended the mosque, but the trustee, Fawzi Haffar, did not know if Mr. Abedi worshiped there.
A senior member of the Muslim community in Manchester and a law enforcement official who requested anonymity said Mr. Abedi had been barred from the mosque in 2015 for expressing his support for the Islamic State, and he came to the attention of intelligence agencies at the time as “a person of interest.”
In raising the threat level, Mrs. May cited information gathered Tuesday in the investigation into the Manchester bombing and said the Joint Terrorism Analysis Center, the body responsible for setting the level, would continue to review the situation.
“The change in the threat level means that there will be additional resources and support made available to the police as they work to keep us all safe,” Mrs. May said.
“I do not want the public to feel unduly alarmed,” she said. “We have faced a serious terrorist threat in our country for many years, and the operational response I have just outlined is a proportionate and sensible response to the threat that our security experts judge we face.”
It was only the third time that Britain had raised the threat level to critical.
The first was on Aug. 10, 2006, after the government foiled a plot to blow up trans-Atlantic airliners with liquid bombs. The second was on June 30, 2007, after two men slammed an S.U.V. into entrance doors at Glasgow Airport and turned the vehicle into a potentially lethal fireball.
After the prime minister’s announcement, Assistant Commissioner Mark Rowley, the head of National Counter Terrorism Policing, said in a statement that “we are flexing our resources to increase police presence at key sites, such as transport and other crowded places, and we are reviewing key events over the coming weeks.”
“I have asked for support from the military to be deployed alongside the police,” Commissioner Rowley added. “This will free up armed officers from certain guarding duties to release our officers to support the wider response.”
As part of their investigation into the Manchester bombing, the police arrested a 23-year-old man outside a supermarket near Mr. Abedi’s home, but it was not immediately clear if that man was connected in some way to the attack.
The terrorist attack was the worst in the history of Manchester, a city of a half-million people, and the worst in Britain since July 7, 2005, when 52 people died, along with four assailants, in coordinated attacks on London’s transit system.
Security experts suggested that the use of an improvised explosive device in Manchester displayed a level of sophistication that implied collaborators — and the possibility that other bombs had been made at the same time.
Chris Phillips, a former leader of the National Counter Terrorism Security Office in Britain, told the BBC: “It has involved a lot of planning — it’s a bit of a step up. This is a much more professional-style attack.”
The Islamic State also claimed responsibility for the March 22 attack near Parliament in which a British man fatally struck four pedestrians on Westminster Bridge before killing a police officer. British authorities say they have also broken up terrorist cells operating in the country.
But investigators fear the Manchester attack indicates a higher level of sophistication, requiring more planning and the possibility of more attacks, prompting the national threat level to be raised.
The bombing came in the final stretch of campaigning before a general election on June 8 in Britain, and the country’s political parties agreed on Tuesday to suspend campaigning. Opposition politicians — Jeremy Corbyn of the Labour Party, Tim Farron of the Liberal Democrats and Nicola Sturgeon of the Scottish National Party — joined Mrs. May in expressing their grief and condolences.
It was unclear what effect the attack might have on the election. Some political experts suggested it would help Mrs. May, who, in her previous role as home secretary, was in charge of Britain’s domestic security and is generally perceived as a tough leader. But difficult questions are already being asked about what security gaps might have abetted the assault, and what could have been done to prevent it.