LONDON - British authorities rolled back the nation's terror threat level from critical to severe on Sunday, hours after a second man was arrested in connection with last week's subway blast in London.
Mark Rowley, assistant commissioner for London's Metropolitan Police, cited "rapid progress" of the investigation into Friday's apparent terrorist attack, but warned that another attack at some point remained "highly likely."
"There is still much more to do," Rowley said. "But this greater clarity and this progress has led (terrorism authorities) to come to the judgment that an attack is no longer imminent."
The Islamic State claimed responsibility for the attack, which it said was carried out by an affiliated unit.
Rowley said searches were continuing at addresses in the London borough of Hounslow and in Surrey, about 30 miles southwest of London. He said investigators were learning more about the preparation of the explosive device used in the attack.
On Saturday, police had announced a “significant” arrest in the case after taking an 18-year-old man into custody in the port city of Dover. The second man, 21, was arrested hours later by detectives from the Metropolitan Police's Counter Terrorism Command unit in the Hounslow.
Officials did not release the names of the two men or any other information about them.
The bomb exploded Friday at 8:20 a.m. aboard a commuter train in the Parsons Green station. The blast and subsequent stampede on the District Line train injured 30 people, including some with burns. None of the injuries was believed to be life-threatening.
The train normally carries commuters, including schoolchildren, from the suburbs. Prime Minister Theresa May on Friday raised the threat level to critical — the highest such level meaning another attack may be imminent — and police called on the public to "remain vigilant."
Images from inside the subway car after the blast showed that the device, concealed in a plastic shopping bag, had been placed in a bucket with wires hanging out of it.
Officials said the bomb was intended to do grave harm to commuters. Analysts said the injuries would have been far worse had the entire device exploded.
“They were really lucky with this one. It could have really become much worse,” said terrorism specialist Magnus Ranstorp of the Swedish Defense University.
Britain has endured four other terrorist attacks this year, which have killed a total of 36 people. The other attacks in London — near Parliament, on London Bridge and near a mosque in Finsbury Park in north London — used vehicles and knives.
In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people. That attack in May also briefly caused the threat level to be set at “critical.”
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