President Donald Trump is marking 17 years since the worst terrorist attack on US soil by visiting the Pennsylvania field that became a September 11 memorial. Mr Trump and his wife Melania are to participate in Tuesday's sombre remembrance in Shanksville, where hijackers crashed a California-bound commercial airliner on September 11, 2001, after the 40 passengers and crew members learned what was happening and attempted to regain control of the aircraft.