British police arrested an 18-year-old man in the ferry departure area of the port of Dover on Saturday on suspicion of being involved in the bombing of a London Underground train and were looking for more potential suspects on the loose.
The man, who has not been named, was arrested on suspicion of committing, preparing or instigating an act of terrorism after Friday's attack in which 30 people were injured at Parsons Green station.
"We're keeping an open mind around whether more than one person is responsible for the attack," Neil Basu of London's Metropolitan Police told reporters.
Dover's ferry terminal was evacuated during the arrest and "a number of items" were recovered, while the teenager is now in custody in London.
The bomb detonated in a packed train carriage on Friday morning with a large explosion followed by what eyewitnesses described as a "fireball". It was Britain's fifth terror attack in six months.
The Islamic State group claimed responsibility.
'One minute to get out'
Police also evacuated a large area around a house in Sunbury near London where a search is ongoing.
Local residents quoted in British media said the owners of the house were elderly foster parents.
"I was in my house with my children and there was a knock at the door from the police," local resident Mojgan Jamali told the Press Association.
"They told me to leave. They said: 'You have one minute to get out of the house and get away'."
Britain's threat level has been raised to "critical", indicating that another attack is feared, and soldiers have been deployed to guard key points to free up police for the investigation.
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 in a televised interview. "At the moment we have one arrest and we have an ongoing operation."
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."
The improvised explosive device in a train at Parsons Green station, a quiet and well-off residential district, failed to detonate fully, according to media reports.
But the blast inflicted flash burns on passengers, and prompted dozens of others to flee in panic.
'Wall of fire'
May announced on Friday that 1,000 troops would be deployed to take on the responsibility for guarding key sites, including nuclear facilities.
The critical alert terror warning was last used after the deadly suicide attack at Manchester Arena, also claimed by IS, in May.
Witnesses described chaos after Friday's explosion.
Charlie Craven told AFP he heard a "massive bang" and "an orange sort of fireball encompassing the whole Tube coming towards you."
Another, Lauren Hubbard, described it as "a wall of fire".
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.
'I could smell the burning'
The remnants of the bomb were examined by forensic scientists but no further details were released. British media reported that it had a timer which had failed to properly detonate.
Several victims were taken to hospital, though health authorities said none were in a serious life-threatening condition.
Only three remained in care on Saturday morning.
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 told AFP.
He was trampled on as panicking passengers stampeded out of the station and his leg was badly cut and bruised.
Four previous attacks in London and Manchester this year claimed the lives of 35 people.
Three of those attacks involved a vehicle ploughing into pedestrians.
