MANCHESTER, England -- The United Kingdom will deploy soldiers to take over guard duties at key sites so elite police can focus on the threat of another terror attack.
While the sight of a member of the armed forces outside concert venues and sporting events may be jarring, in every case the soldiers will be under the command of police officers, Prime Minister Theresa May said.
The presence of troops comes as the U.K. raised its threat level to "critical," its highest. Such a level indicates another attack could be "imminent."
Authorities have been trying all day to determine whether Salman Abedi, the 22-year-old British-born man who they say carried out the attack outside the Ariana Grande concert at Manchester Arena on Monday night, acted alone.
They haven't ruled out the possibility that others might be involved.
"The work undertaken throughout the day has revealed that it is a possibility that we cannot ignore, that there is a wider group of individuals linked to this attack," May said in a speech Tuesday.
As a result, the U.K. has opted to escalate its security presence and has activated a plan called Operation Temperer.
As part of the plan, soldiers will replace police officer at key sites, May said. She didn't elaborate what kinds of sites. They are likely to include railway stations and airports.
"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe," she said.
The suicide blast in Manchester took place at the end of a concert by pop star Ariana Grande, and killed 22 people. Another 59 were injured.
And with the high-profile FA Cup final coming up this weekend, officials aren't taking any chances.
"I do not want the public to feel unduly alarmed." May said.
The response, she added, "is a proportionate and sensible response to the threat that our security experts judge we face."
The last time the country deployed troops was in 2003 in reaction to a plot by a suspected al-Qaida cell to bring down a British airliner. But it was only at Heathrow Airport.
Likewise, there have only been two previous instances when the threat level was raised to "critical." Both were in response to a specific plot or attack.
In 2007, a man tried to ram a burning four-wheel-drive, loaded with gas canisters, through the main terminal of Glasgow Airport.
In 2006, officials thwarted a plot to simultaneously blow up as many as 10 jets leaving Britain for the U.S.
The blast Monday night marked the deadliest terror attack on British soil since the 2005 London bombings.
A powerful explosion shook part of the cavernous Manchester Arena late Monday as concertgoers streamed out after the American pop star's last song.
ISIS claimed responsibility for the attack but offered no evidence to support its claim.
ISIS said on its Telegram channel that a "soldier of the caliphate" was able to "plant explosive devices" at the arena, a U.S. counter-terrorism source said.
An 8-year-old girl and an 18-year-old woman have been named as the first victims of the attack, which has drawn condemnation and horror from around the world as a heinous assault targeting children.