Manchester attack: UK raises terror threat as bomber details emerge

Prime Minister Theresa May announced Tuesday night that Britain's threat level had been raised from "severe" to "critical," and warned that a "further attack may be imminent."

The change in status has led to speculation that that Abedi may not have been acting alone and police are investigating whether Abedi belonged to a wider network. Rudd said that it was "likely, possible, that he wasn't doing this on his own."

Police have conducted a series of raids since the attackĀ and arrested three men in south Manchester on Wednesday. Another 23-year-old man was arrested on Tuesday.

Up to 3,800 military personnel have been made available across Britain following the attack, the Home Secretary announced, and almost 1,000 are now deployed.

London's Metropolitan Police service announced that military personnel would guard "key locations" as part of what's been called "Operation Temperer," and soldiers were seen at Buckingham Palace and extra police at train stations on Wednesday morning.

Police in London said a man with a knife was arrested near Buckingham Palace Wednesday morning, but they stressed that the incident was not terrorism-related.

"I would expect this to be temporary but we will keep a close eye to see how long we need them for and when it's appropriate we end Operation Temperer and go back to our different levels," Rudd said.

Protected sites in London also include Downing Street, embassies and the Palace of Westminster. The deployments would free up armed police officers to carry out patrols of the city, London police said in a statement.

The British parliament said that it was closing its doors to non-passholders and canceled all its tours, events and banquets.