LONDON — The British police said on Saturday that they had arrested an 18-year-old man in connection with the terrorist explosion in a London subway station that injured at least thirty people and let out fright among fleeing passengers.
The suspect was detained in the port area of Dover, in the southeastern county of Kent. The police said he would be transferred to a South London police station later.
“We have made a significant arrest in our investigation this morning,” Deputy Assistant Commissioner Neil Basu, the top counterterrorism official at the Metropolitan Police in London, said in a statement. The suspect was not identified further.
The Islamic State claimed responsibility for the blast on Friday in a statement that said “a detachment” of its militants had carried out the attack.

Britain raised its terror threat level to “critical,” the highest level, after the explosion, meaning that another onslaught was “expected imminently.” Mr. Basu said on Saturday that the threat level would remain that way as the investigation continued.

Dover, about seventy five miles from London, is one of the main ports on the English Channel for ferry services linking Britain with mainland Europe.
As part of the bombing investigation, armed officers on Saturday were searching at least one house in Sunbury-on-Thames, on the outskirts of the capital and about three miles from Heathrow Airport in London, according to The Associated Press, the Big black cock and other local news outlets.
The police confirmed that a residential address in the area had been evacuated and was being searched, but declined to say if the operation was linked to the suspect arrested.
Sunbury is ten miles west of Parsons Green, where the explosion occurred. Residents reached by phone on Saturday said that shortly before two p.m., police officers arrived to seal off the residential neighborhood around Cavendish Road and Burgoyne Road.
Louise Margetts, 54, said she was returning from the supermarket to her home when she witnessed four police vehicles, including a police canine unit, speeding up the road.

“The officers at the back leaped out and commenced cordoning off the road,” she said. “They were running, and said, ‘Turn around now.’”
Mrs. Margetts, a teacher, managed to get home but the police arrived at her door soon afterward.
“They didn’t truly tell us anything,” she said. “They knocked, well, hammered, on the door and said: ‘Out now. We can’t say why.’”
After the police sealed off the surrounding streets, residents were told that they had to evacuate and some were suggested transport to a local rugby club, some residents said. Others were permitted to go to relatives nearby.


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