LONDON — A second man has been arrested over Friday’s bomb attack on a London underground train that injured 30 people, police said on Sunday, as Britain remained on its highest level of alert with soldiers helping provide security.

The 21-year-old man was detained under Britain’s Terrorism Act in the west London suburb of Hounslow just before midnight on Saturday, the Metropolitan Police said in a statement.

Police arrested an 18-year-old man in the port of Dover earlier on Saturday and then raided a property in Sunbury, a town near London and about four miles (six km) from Hounslow.

The home-made bomb shot flames through a train carriage packed with commuters at west London’s Parsons Green Tube station but apparently failed to detonate fully.

Islamic State claimed responsibility as it has for other attacks in Britain this year, including two in London and one at a concert by American singer Ariana Grande in Manchester in May.

Intelligence officials say there is scant evidence the militant group was behind the attacks.

Prime Minister Theresa May put Britain on its highest security level of “critical” late on Friday, meaning another attack might be imminent. Soldiers and armed police were deployed to strategic locations such as nuclear power plants.

The first man arrested on Saturday was detained in the departure lounge of the port of Dover in what police said was a “very significant” step.

The first arrest on Saturday took place at the Dover ferry terminal — a main link to Europe. A “number of items” were recovered during the operation and the teenager is now in custody in London, officers said.

Police had earlier raided a home in Sunbury, a town west of London. Local residents quoted in British media said the owners of the house were elderly foster parents.

The last time Britain was put on “critical” alert was after a suicide bomber killed 22 people at the Ariana Grande concert.

On that occasion, the threat level remained at the highest setting for four days while police raced to establish if the bomber had worked alone or with the help of others. Prior to that it had not been triggered since 2007.

Home Secretary Amber Rudd said Saturday that police had made “very good progress” in their enquiries, while appearing to dispute claims by US President Donald J. Trump that a “loser terrorist” behind the attack was known to Scotland Yard.

“It’s much too early to say that,” Mr. Rudd said in a televised interview.

Mr. Trump’s claims, made Friday on Twitter, had already garnered a terse rebuke from Britain’s Ms. May, who said: “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

Ms. May also announced on Friday that a thousand troops would be deployed to take on the responsibility for guarding key sites, including nuclear facilities.

‘FIREBALL’
The improvised device at Parsons Green, a quiet and well-off residential district, failed to detonate fully.

But the blast inflicted flash burns on passengers, and prompted dozens of others to flee in panic.

Twitter user @Rrigs posted pictures of a white bucket smouldering on the train and described how a “fireball flew down carriage and we just jumped out open door.”

The bucket, which was inside a frozen food bag, looked like the type used by builders and there appeared to be cables coming from it.

Louis Hather, 21, had been traveling to work and was three carriages down from where the explosion took place.

“I could smell the burning. Like when you burn plastic,” he told AFP.

He was trampled on as panicking passengers stampeded out of the station and his leg was badly cut and bruised.

The bomb’s remnants were examined by forensic scientists but no further details were released.

Several victims were taken to hospital, though health authorities said none were in a serious life-threatening condition. — Reuters and AFP

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