WASHINGTON A U.S. passenger plane carrying 64 people crashed into Washington DC's chilly Potomac River after colliding mid-air with a military helicopter on a nighttime training exercise Wednesday, prompting a major emergency response and the grounding of all flights. The plane had been about to land at nearby Reagan National Airport after flying from Kansas. American Airlines, whose subsidiary operated the flight, said "there were 60 passengers and four crew members on board the aircraft." A U.