LONDON -- British police have made an apparent breakthrough in subway bombing investigation with what they are calling a "very significant" arrest, but the country remains 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. Residents said they were evacuated immediately as police established a huge cordon and imposed a no-fly zone above the property being searched.
RELATED: BART beefs up security after London subway explosion that left at least 29 injured
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.
Rudd said Saturday it was "good fortune" that the bomb on the District Line train did not do more damage. She said it was a "serious" improvised explosive device that could have caused much more harm.
Rudd said she has briefed Prime Minister Theresa May on developments in the attack Friday that left 29 people injured.
PHOTOS: Scene of explosion on London train
Evacuations ordered after 'significant arrest' made in London terror attack