In the most devastating terrorist onslaught ever waged against the United States, knife-wielding hijackers crashed two airliners into the World Trade Center on Tuesday, toppling its twin 110-story towers. The deadly calamity was witnessed on televisions across the world as another plane slammed into the Pentagon, and a fourth crashed outside Pittsburgh.

?Today, our nation saw evil,? President Bush said in an address to the nation Tuesday night. He said thousands of lives were ?suddenly ended by evil, despicable acts of terror.?

Said Adm. Robert J. Natter, commander of the U.S. Atlantic Fleet: ?We have been attacked like we haven?t since Pearl Harbor.?

Establishing the U.S. death toll could take weeks. The four airliners alone had 266 people aboard and there were no known survivors. At the Pentagon, about 100 people were believed dead.

In addition, a firefighters union official said he feared an estimated 200 firefighters had died in rescue efforts at the trade center ? where 50,000 people worked ? and dozens of police officers were believed missing.

?The number of casualties will be more than most of us can bear,? a visibly distraught Mayor Rudolph Giuliani said.

No one took responsibility for the attacks that rocked the seats of finance and government. But federal authorities identified Osama bin Laden, who has been given asylum by Afghanistan?s Taliban rulers, as the prime suspect.

Aided by an intercept of communications between his supporters and harrowing cell phone calls from at least one flight attendant and two passengers aboard the jetliners before they crashed, U.S. officials began assembling a case linking bin Laden to the devastation.

U.S. intelligence intercepted communications between bin Laden supporters discussing the attacks on the World Trade Center and Pentagon, according to Utah Sen. Orrin Hatch, the top Republican on the Senate Judiciary Committee.

The people aboard planes who managed to make cell phone calls each described similar circumstances: They indicated the hijackers were armed with knives, in some cases stabbing flight attendants. The hijackers then took control of the planes.

At the World Trade Center, the dead and the doomed plummeted from the skyscrapers, among them a man and woman holding hands.

Shortly after 7 p.m., crews began heading into ground zero of the attack to search for survivors and recover bodies. All that remained of the twin towers by then was a pile of rubble and twisted steel that stood barely two stories high, leaving a huge gap in the New York City skyline.

Emergency Medical Service worker Louis Garcia said initial reports indicated that bodies were buried beneath the two feet of soot on streets around the trade center.

?A lot of the vehicles are running over bodies because they are all over the place,? he said.

Said National Guard member Angelo Otchy of Maplewood, N.J., ?I must have come across body parts by the thousands. I came across a lady, she didn?t remember her name. Her face was covered in blood.?

Later on, Bush addressed the nation.

?Freedom itself was attacked this morning and I assure you freedom will be defended,? said Bush, who was in Florida at the time of the catastrophe. As a security measure, he was shuttled to a Strategic Air Command bunker in Nebraska before leaving for Washington.

?Make no mistake,? he said. ?The United States will hunt down and pursue those responsible for these cowardly actions.?

Florida state officials said they had no evidence that specific government buildings in Florida had been targeted, but measures were taking Tuesday to ensure statewide security.

NASA went on a maximum level alert, sending nearly all of its 12,000 workers home and closing its main spaceport at the Kennedy Space Center.

Security was increased for Gov. Jeb Bush, who said it was ?appropriate? to evacuate the Capitol ?given the randomness of the attack and the fact it is a high profile building.?

Disney World, one of the world?s most popular vacation destinations, closed its theme parks and sent employees home, spokeswoman Rena Callahan said. Universal Studios, SeaWorld Orlando and Busch Gardens in Tampa also closed. All Walt Disney theme parks would reopen Wednesday, the company said.

Officials at MacDill Air Force Base in Tampa, home to U.S. Central Command, responsible for military operations in the Middle East, said it was in ?a state of increased security vigilance,? in response to a directive from the Joint Chiefs of Staff.