Heathrow said a security operation after a pepper-spray attack in the Terminal 3 car park disrupted travel, forcing road and rail closures while emergency services responded.Police arrested one man on suspicion of assault, and Commander Peter Stevens said investigators were focusing on a group who knew each other and that the incident was not being treated as terrorism.Ambulance crews treated 21 people for gas exposure and admitted five to hospital, National Rail confirmed train services had resumed but delays remained on the Elizabeth and Piccadilly lines and police kept a heightened presence.