An 18-year-old man was reportedly arrested on Saturday in connection with the blast on a packed underground train at a London station on Friday.
The Islamic State earlier claimed responsibility for the improvised explosive device (IED) explosion which left 25 injured.
UK Prime Minister Theresa May said was "clearly intended to cause significant harm".
The explosion, on a District Line Tube in the British capital's southwest, occurred as the train pulled into Parsons Green station. Commuters said they heard a bang and saw a fireball inside the carriage when the "bucket bomb" exploded.
Several people suffered facial burns, some reportedly were even trampled in the rush to escape. But none of the injuries were life threatening. Pictures of a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor of the train went viral on social media.
Investigators were examining what appears to be a circuit board recovered from the scene, news reports said. Scotland Yard said it was a terror incident and they "assess" an IED triggered the blast.