NEW YORK/WASHINGTON (Reuters) - Three hijacked planes crashed into U.S. landmarks on Tuesday, destroying New York's mighty twin towers and plunging the Pentagon in Washington into flames in an unprecedented assault on key symbols of U.S. military and financial power. Loss of life was expected to be catastrophic from the collapse of the giant towers of the World Trade Center where some 50,000 people work. The two 110-story towers collapsed one at a time in a huge cloud of smoke and fire two hours after the initial impacts.