Three hijacked planes crashed into major U.S. landmarks this afternoon, destroying both of New York's mighty twin towers, hitting the Pentagon in Washington and plunging the United States into unprecedented chaos and panic. Loss of life was expected to be catastrophic from the collapse of the giant towers of the World Trade Center where many thousands of people work. The two enormous edifices both fell in a huge cloud of smoke and fire two hours after the initial impacts. New York Mayor Rudolph Giuliani said there had been a tremendous number of lives lost in the attacks on his city. Another plane crashed near Pittsburgh. It was not clear if this was another attempted hijacking. Hospitals in New York were overwhelmed with patients as a massive cloud billowed into the blue skies over Manhattan where the city skyline had been dramatically and permanently altered. «Hundreds of people are burned from head to toe, said Dr. Steven Stern at St. Vincent's Hospital in the Greenwich Village neighborhood of lower Manhattan. The whole of lower Manhattan is coated in half an inch of dust, Reuters reporter Daniel Sternoff said. President George W. Bush cut short a visit to Florida and rushed back to Washington to face the greatest crisis of his young presidency. He called the deliberate aerial assaults an apparent terrorist attack, and ordered a full-scale investigation. Early speculation about the source of the attack centered on Saudi-born guerrilla leader Osama Bin-Laden. It was the most dramatic and deadly attack on the U.S. mainland in modern history. The attacks forced the evacuation of all government buildings in Washington, including the White House and other tall buildings around the country, cut cell phone communications on the East Coast and grounded all commercial planes in the United States. Early reports said all three planes used in the attacks were hijacked, one of them from Boston and one from Washington. It was not immediately known who flew the planes and what happened to them. The day of horror began around 9 a.m. (1300 GMT) in New York when the first plane plowed into the south tower of New York's World Trade Center, as thousands of workers were streaming into the building to begin their day. It opened a huge hole near the top of the building. Two hours later, the whole building in which thousands of people collapsed on itself in a huge cloud of smoke and fire. TV stations caught the second plane plowing into the second of the twin towers, exploding in a fire ball a few minutes after the first impact. That building caved in about an hour after the first. Shortly afterwards, a third plane crashed into or near the Pentagon in Washington, throwing people off their feet inside the building and setting off a massive fire. Amid confusion, news organizations reported another explosion at the State Department but this was later denied. Other reports spoke of another hijacked plane heading toward the capital. All rnment buildings including the White House and the Capitol and the CIA were evacuated. The Federal Aviation Authority grounded all planes in the United States, an unprecedented step. It's clear that this is terrorist-related, we're not sure who is responsible, one official said of the Pentagon attack. There was no advance warning of this, the official said on condition of anonymity. One of the planes that crashed into the World Trade Center was American Airlines' Flight 11 from Boston to Los Angeles, said Lori Bassani, spokesperson for American's flight attendants union. Bassani said if full, the flight on the Boeing 767 would carry 158 passengers. That would include two pilots and nine or 10 flight attendants, she said. She said she did not know how many people were on board.