Hijacked Planes Destroy Twin Towers, Hit Pentagon By Alan Elsner, National Correspondent 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 (news - web sites) 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. President Bush (news - web sites) 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. Bush cut short a visit to Florida and rushed back to Washington to face the greatest crisis of his young presidency. New York Mayor Rudolph Giuliani (news - web sites) said there had been a ''horrendous 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, forcing the evacuation of key buildings, closing markets, schools and even theme parks. Sirens screamed as terrified people rushed through the streets seeking safety. American Airlines said two of its jets carrying a total of 156 people were lost. United Airlines said one of its planes had crashed in Pennsylvania with unknown loss of life and another was missing. As international flights were diverted to Canada, the Federal Aviation Administration (news - web sites) shut down all flights in the United States and said it had yet to account for a number of planes. HOSPITALS OVERWHELMED Palestinians in the occupied West Bank and gunmen at refugee camps in Lebanon fired into the air to celebrate news of the attacks. 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 across the country, cut cell phone communications on the East Coast and grounded all commercial planes in the United States. World leaders expressed shock and horror and foreign financial markets fell sharply on news of the attacks. The London FTSE index plummeted 5.7 percent, while oil prices spiked up by 3 percent. U.S. markets were closed. 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. 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 afterward, 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. Photos Reuters Photo Amid confusion, news organizations reported another explosion at the State Department, but that 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 (news - web sites) were evacuated. The FAA 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. The other was flight 77, a Boeing 757, operating from Washington Dulles to Los Angeles. United said flight 93 from Newark to San Francisco, a Boeing 757, had crashed south of Pittsburgh. The airline said it had no other details of how many people were on board. United also said it was ``deeply concerned'' about another flight, No. 175, which departed from Boston. The airline said it had no details of that flight. 'A NATIONAL TRAGEDY' Speaking before the full horror had unfolded, Bush said: ''Terrorism against our nation will not stand. 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 (news - web sites), 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. The attacks took place near the anniversary of the 1978 Camp David accords that led to peace between Israel and Egypt. Bin Laden, a Saudi millionaire and Islamic militant, believed to be in exile in Afghanistan (news - web sites), 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 the renegade Saudi had warned three weeks ago of an ''unprecedented attack'' on U.S. interests. Washington has offered a $5-million reward for his capture. George Tenet, director of the CIA, said this week the tall, thin Saudi was the most immediate and serious threat to U.S. security. Beside the embassy bombings, U.S. officials link bin Laden to last year's bombing of a U.S. Navy (news - web sites) ship in Yemen and with foiled plots in the United States and Jordan at the turn of the millennium. ``Since 1998, bin Laden has declared all U.S. citizens legitimate targets of attack,'' he said. 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 (news - web sites) 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.