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Britain raised its national security threat level to critical from severe on Friday, meaning an attack is expected, after a bomb on a London commuter train injured 29 people.

Prime Minister Theresa May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.

READ MORE: What we know about London subway explosion, U.K.’s 5th terrorist incident in 2017

“For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said.

British police are chasing down the people they suspect of being behind the bomb, the country’s most senior counter-terrorism officer said.

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WATCH: At least 22 injured in ‘terrorist’ incident on London subway train

1:39 At least 22 injured in ‘terrorist’ incident on London subway train At least 22 injured in ‘terrorist’ incident on London subway train

Mark Rowley said officers were sifting through surveillance footage and examining the remains of the device.

Twenty-nine people were injured after a home-made bomb on a packed rush-hour commuter train engulfed a carriage in flames, but apparently failed to fully explode, in Britain’s fifth major terrorism incident this year.