NEW YORK (AP) – 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 George W. 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,” 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.

“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.”

More than nine hours after the U.S. attacks began, explosions could be heard north of the Afghan capital of Kabul, but American officials said the United States was not responsible.

“It isn’t us. I don’t know who’s doing it,” Pentagon spokesman Craig Quigley said.

Officials across the world condemned the attacks but in the West Bank city of Nablus, thousands of Palestinians celebrated, chanting “God is Great” and handing out candy. The United States has become increasingly unpopular in the Mideast in the past year of Israeli-Palestinian fighting, with Washington widely seen as siding with Israel against the Arab world.

At the Pentagon, the symbol and command center for the nation’s military force, one side of the building collapsed as smoke billowed over the Potomac River. Rep. Ike Skelton, briefed by Pentagon officials, said, “There appear to be about 100 casualties” in the building.

The first airstrike occurred shortly before 8:45 a.m. EDT. By evening, huge clouds of smoke still billowed from the ruins. A burning, 47-story part of the World Trade Center complex – already evacuated – collapsed in flames just before nightfall.

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.”

For the first time, the nation’s aviation system was completely shut down as officials considered the frightening flaws that had been exposed in security procedures. Financial markets were closed, too.

Top leaders of Congress were led to an undisclosed location, as were key officials of the Bush administration. Guards armed with automatic weapons patrolled the White House grounds and military aircraft secured the skies above the capital city. National Guard troops appeared on some street corners in the nation’s capital.

Evacuations were ordered at the tallest skyscrapers in several cities, and high-profile tourist attractions closed – Walt Disney World, Mount Rushmore, Seattle’s Space Needle, the Gateway Arch in St. Louis.

The Federal Reserve, seeking to provide assurances that the nation’s banking system would be protected, said it would provide additional money to banks if needed.

In Afghanistan, where bin Laden has been given asylum, the nation’s hardline Taliban rulers rejected suggestions he was responsible.

Bin Laden came to prominence fighting alongside the U.S.-backed Afghan mujahedeen – holy warriors – in their war against Soviet troops in the 1980s. But former followers say he turned against the United States during the 1991 Gulf War, seething at the deployment of U.S. troops in Saudi Arabia during the Gulf War campaign to oust Iraq from Kuwait. He has repeatedly called on Muslims worldwide to join in a jihad, or holy war, against the United States.

Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper, said he received a warning from Islamic fundamentalists close to bin Laden, but had not taken the threat seriously. “They said it would be a huge and unprecedented attack, but they did not specify,” Atwan said in a telephone interview in London.

Eight years ago, the World Trade Center was a terrorist target when a truck bomb killed six people and wounded about 1,000 others. Just the death toll on the planes alone surpassed the 168 people killed in the 1995 bombing of the federal building in Oklahoma City.

A grim-faced President Bush asked the nation to find comfort in Scripture as he mourned the deaths of thousands of Americans in Tuesday’s atrocities and vowed to avenge their killings.

“Today, our nation saw evil,” he said.

In his first prime-time Oval Office address, Bush said the United States would find and punish “those behind these evil acts,” and any country that harbors them.