(Bloomberg) —The British military will guard music and sports events in an unprecedented security operation as police hunt for potential accomplices of the suicide bomber who killed 22 people at a Manchester pop concert.

After meeting with her top security officials, Prime Minister Theresa May announced intelligence analysts had raised the U.K. terrorism threat level from “severe” to “critical” — the highest level — for the first time in a decade.

“This means that their assessment is not only that an attack remains highly likely but that a further attack may be imminent,” the premier said in a statement to media inside her official residence in London. Police asked for back up from the army, she said.

The worst terrorist incident on U.K. soil since 2005 comes just two weeks before a general election. All parties have suspended campaigning indefinitely, with no activities planned for Wednesday. The deployment of troops to protect against further attacks will make for a tense backdrop to the final days before the June 8 vote.

May visited Manchester — in Northern England– on Tuesday and vowed to fight the “ideology” behind the attack on a concert packed with children and teenage fans.

After returning to London to chair the government’s emergency COBRA committee of security chiefs, May updated the press on the progress of the police investigation and the threat of further attacks.

British-Born

She confirmed that Salman Ramadan Abedi, the 22 year-old suicide bomber who carried out the attack at Manchester Arena on Monday that also injured 59, was born and raised in the U.K. Security services are working to establish whether he was part of a larger terrorist cell, she said.

“The work undertaken throughout the day has revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack,” May said. Police arrested a 23-year-old man in south Manchester on Tuesday as a suspected accomplice.

Military reinforcements will be used to defend potential targets across the U.K., allowing the police to “significantly increase the number of armed officers on patrol in key locations,” May said. “You might also see military personnel deployed at certain events, such as concerts and sports matches, helping police to keep the public safe.” The military will be under the command of police chiefs.

It is the first time that Operation Temperer — the U.K.’s plan for deploying up to 5,000 troops to guard key sites from terrorism — has been put into action.

Transport Hubs

London’s Metropolitan Police announced security will be stepped up at transport hubs and other crowded places, while large public events could be canceled. More armed police will be on patrol to guard high profile soccer and rugby matches at Wembley and Twickenham stadiums in the capital in the coming days.

Security services will work with organizers of major events over the next few weeks to “make sure decisions are taken that events only go ahead when it’s sensible and safe to do so,” said U.K. counter-terrorism police chief Mark Rowley.

May said she didn’t want the public to be “alarmed,” insisting the security response was “proportionate and sensible.”

The Manchester attack is the latest to traumatize Europe over the past two years and is sure to be discussed at this week’s meeting of NATO leaders in Brussels. As Britain prepares to leave the EU, the bombing underscored the possible threat of losing access to the bloc’s intelligence and security collaboration framework.

May, who served as home secretary from 2010 until becoming premier last year, has an intimate knowledge of the work of police and security services, and the many terrorist plots that have been foiled. The Manchester attack stands out from recent incidents which have been more low-tech, relying on knives and cars.