U.K. Police Arrest Men in Connection with London Bombing at Parsons Green

An 18-year-old man was detained in Dover’s port area; a 21-year-old man was later arrested in Hounslow

British police on Saturday detained an 18-year-old man in the English Channel town of Dover in connection with the rush-hour bombing of a London subway train that left 30 injured, calling the arrest “significant” and saying the investigation was moving quickly.

The man was arrested in the port area of the town, a major hub for ferry transport between the U.K. and France about 80 miles from the capital.

Authorities were urgently tracking down leads in what they were treating as a terrorist attack, with London police saying they were searching a residential address in Surrey, southeast of the capital, and had evacuated the area as a precautionary measure.

It was unclear if the man arrested was the person believed responsible for the attack and a key question was the extent of any network behind the bombing.

Police late Saturday also arrested a 21-year-old man in Hounslow in west London, London’s Metropolitan Police said in a statement released Sunday morning.

Metropolitan Police Commissioner Cressida Dick said the investigation was fast-moving and that police were making “considerable progress.”

According to the SITE Intel Group, Islamic State claimed responsibility via its news agency, Amaq, saying it was carried out by a “detachment.” There was no independent indication of the group’s role.

In the Friday morning bombing, a homemade device partially exploded on a train at the Parsons Green station in West London, leaving many of those injured with flash burns and sending people screaming in panic. A photo posted on social media showed personal belongings and a bucket with an item on fire inside on the floor of a train carriage.

In another separate statement, Islamic State said it had deployed several explosives but only one detonated. That too couldn’t be independently verified.

Police have spoken to 45 witnesses and have received 77 images and videos from the public, police said.

The threat level remained at critical after Prime Minister Theresa May said Friday it had been raised to the highest level on a five-rung scale. Mrs. May said the change means security officials’ assessment was “that a further attack may be imminent.”

Police stepped up security in the wake of the attack and extra armed officers were deployed around the capital.

“They are there to keep us safe,” London Mayor Sadiq Khan said. “London will never be intimidated by terrorism. We will always defeat those who seek to harm us and destroy our way of life.”

The U.K. has faced five terrorist attacks in the last six months, underscoring the country’s vulnerability to large-scale assaults, even with some of the most sophisticated security services in the world. Home Secretary Amber Rudd said it was “much too early to say” whether any perpetrator was already known to authorities.

The U.K. has beefed up counterterrorism measures, such as requiring teachers, including at preschools, to report extremist leanings or behavior by students to police.

Maj. Gen. Chip Chapman, former head of counterterrorism at the Ministry of Defense said that in past decades the country faced a major threat from Irish nationalists. “All this stuff about scale and tempo of terrorism that we’ve never seen before is just not true,” Mr. Chapman said.

Mr. Chapman, who was the senior British military adviser to U.S. Central Command and is not involved in the current investigation, said that given the relative skill it would have taken to put together the bomb it was likely the attacker was working with others, at least some of them among the roughly 20,000 suspected extremists security services have tracked.

“It’s very rare you’ve got someone who self-radicalized who puts a bomb together with that sophistication without other people knowing,” he said.

The last time the threat assessment was raised to its highest level was in May, when police suspected the suicide bomber who killed 22 people at a pop concert in the northwestern British city of Manchester may have had an accomplice. Police reduced its terrorism-threat level back to severe less than a week after the attack after making arrests.