British Prime Minister Theresa May has raised the country's threat level from severe to critical following an attack at a London train station on Friday. The elevated alert level means "an attack is expected imminently."

The explosion at the Parsons Green London Underground train station was caused by an improvised explosive device similar to those used in attacks in the past. The blast injured at least 29 people, many of whom had "flash burn" injuries.

May's decision to raise the threat level means the British army will be deployed on the streets to take over certain duties normally carried out by the police. The last time the threat level was raised to critical was in May following an attack at an Ariana Grande concert in Manchester.

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Meanwhile, the Islamic State of Iraq and Syria (ISIS) has claimed responsibility for the attack. In a statement on its official channel on the messaging app Telegram, the group said the attack was "carried out by a detachment of the Islamic State."

A subsequent statement said attackers "managed to plant a number of explosive devices, and detonated one of them." Police have not announced the discovery of other explosives at the Parsons Green station.

A U.S. law enforcement source told CBS News senior investigative producer Pat Milton the device used in the attack was a white plastic bucket with a black cloth on top, with wires believed to be Christmas lights coming out of the bucket. The device included a timer and was carried in an insulated shopping bag, the source said.