The bomb detonated in a packed train carriage Friday morning with a large explosion followed by what an eyewitness described as a “fireball”.

Later in the day, the British government lowered the country’s official terrorist threat level from “critical” to “severe”, meaning authorities no longer believe an attack is imminent.

Home Secretary Amber Rudd said the easing of the alert indicated that police and security services are making “good progress” in the sprawling investigation into the attack.

It was Britain’s fifth terror attack in six months – a series that has claimed 35 lives.

Islamic State claimed responsibility.

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.

Britain’s terror threat was raised on Friday to “critical”, indicating that another attack is feared, and soldiers have been deployed to guard key points to free up police for the investigation.

The critical warning was last used after a deadly suicide attack at Manchester Arena, also claimed by IS, in May.

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 Trump that a “loser terrorist” behind the attack was known to Scotland Yard.

“It’s much too early to say that,” Rudd said.

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

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

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

Watch: UK threat level raised to ‘critical’

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 travelling to work and was three carriages down from where the explosion took place.

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

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.

Additional reporting by Associated Press