Three planes have crashed into major U.S. landmarks in New York and Washington, destroying New York's mighty twin towers and hitting the Pentagon in deadly attacks on the United States. Loss of life was feared to be catastrophic and could number in the hundreds or even thousands. Both of the twin towers of New York's World Trade Center were struck by planes deliberately flying into them at the start of the workday on Tuesday. Tens of thousands of people work in the buildings ever day. Shortly afterwards, a third plane crashed into or near the Pentagon in Washington, throwing people off their feet inside the building and causing a big fire. That building, the White House, the Capitol, the Supreme Court and other key government buildings were evacuated as the nation ground to a virtual halt and panic spread throughout the country. Authorities grounded all planes in the United States, an unprecedented step. President George W. Bush called the deliberate aerial assaults an "apparent terrorist attack," and vowed to hunt down those responsible. Early reports said all three planes used in the attacks were hijacked, two of them from Boston. It was not immediately known who flew the planes and what happened to them.