Tuesday September 11 11:35 AM ET

By Alan Elsner, National Correspondent

NEW YORK/WASHINGTON (Reuters) - Three hijacked planes crashed into U.S. landmarks on Tuesday, destroying both of 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 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 assault on his city.

The attacks, the worst on the U.S. mainland in modern history, plunged the country into chaos and panic, paralyzing communications, closing markets and forcing the evacuation of key buildings.

Another plane crashed south of Pittsburgh. It was not clear if this was another attempted hijacking. As international flights were diverted to Canada, the Federal Aviation Authority said it had yet to account for a number of planes.

President Bush cut short a visit to Florida and rushed back to Washington to face the greatest crisis of his young presidency.

'APPARENT TERRORIST ATTACK'

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.

Palestinian gunmen at refugee camps in Lebanon fired into the air to celebrate news of the attacks on major U.S. landmarks and government offices.

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.

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. 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.

HUGE HOLE IN TWIN TOWER

It opened a huge hole near the top of the building. Two hours later, the whole building in which thousands of people work, 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 government 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.

'A NATIONAL TRAGEDY' 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.

``Terrorism against our nation will not stand,'' Bush said before leaving Florida for the capital.

``Today we've had a national tragedy. Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country,'' he said, speaking before schoolchildren, teachers and parents at Emma E. Booker Elementary School, where he had planned to talk about education.

``I have spoken to the vice president, to the governor of New York, to the director of the FBI, and have ordered that the full resources of the federal government go to help the victims and the families and to conduct a full scale investigation to hunt down and to find those folks who committed this act,'' said Bush.

He called for a moment of silence. ``May God bless the victims, their families and America,'' Bush said, his voice breaking with emotion.

Bin Laden, a Saudi millionaire and Islamic militant, believed to be in exile in Afghanistan, was blamed for the 1998 bombings of the U.S. embassies in Kenya and Tanzania in which 224 people died.

An Arab journalist with access to Bin Laden told Reuters in London that renegade Saudi had warned three weeks ago of an ''unprecedented attack'' on U.S. interests.

The previous worst act of terrorism in the United States was the 1995 bombing of the federal building in Oklahoma City in which 168 people died. Timothy McVeigh was executed for that attack earlier this year.

A previous bombing of the World Trade Center in 1993 resulted in six deaths and hundreds of injuries.

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