LONDON (AP) -- London police say a second man has been arrested in connection with the London subway attack.
Police said Sunday that a 21-year-old man was arrested late Saturday in Hounslow in west London under the Terrorism Act. He is being questioned at a south London police station but has not been charged or identified.
Two men are now in custody for possible roles in the bombing attack on a rush-hour subway train Friday that injured 29 people in London. An 18-year-old man was arrested Saturday in Dover, where ferries leave for France.
The two arrests indicate authorities believe the attack at the Parsons Green station was part of a coordinated plot, not the act of a single person.
Britain's terror threat level remains at "critical" -- the highest level -- meaning that authorities believe another attack is imminent.