By GREGORY KATZ
LONDON — British police made an apparent breakthrough Saturday in the race-against-time subway bombing investigation with what they called a “very significant” arrest, but the country remained on a “critical” alert, meaning that another attack is judged imminent.
Police arrested an 18-year-old man in the port of Dover — the main ferry link to France — and then launched a massive armed search in the southwestern London suburb of Sunbury in which they evacuated residents, established a huge cordon and imposed a no-fly zone above the property being searched.
Police did not say that they had nabbed the man believed to have planted the bomb that partially exploded on a crowded London subway train Friday morning, but Home Secretary Amber Rudd and others said the arrest was of major importance.
The man is being held under the Terrorism Act and has been brought to London for questioning. His identity is a closely guarded secret and police have implored the press not to speculate while the inquiry unfolds. Authorities would not say if they thought the man was trying to flee to France on a Dover ferry.
It’s clear that Britain’s police and security services are still worried. Hundreds of soldiers patrolled public areas Saturday, freeing up police for the bombing investigation. Rudd said the country’s terror threat level — which was raised Friday night to the highest possible level — will stay there until the independent Joint Terrorism Analysis Center is convinced the threat of imminent attack has…