"Today our nation saw evil, the very worst of human nature," Mr. Bush said.

Earlier, at a Pentagon briefing, Army Gen. Hugh Shelton, chairman of the Joint Chiefs of Staff, said he would not discuss any possible military response, "but make no mistake about it, our armed forces are ready."

The suicide missions by the terrorists left Americans deeply shaken, as they watched the most potent symbols of the nation's financial and military might disintegrate, sickeningly, in billows of black smoke.

A fourth airliner also crashed Tuesday, going down in a field near Pittsburgh. Some authorities speculated that it had been preparing to hit the presidential retreat at Camp David, Md.

A woman on the streets of New York captured the enormity in a single word: "Armageddon."

Said a police officer, "New York is crying."

By nightfall, rescue workers, police and firefighters pressed their desperate search for survivors, including some using cellphones calling for help. Hospitals were overflowing, and authorities were treating some of the injured at Liberty State Park in New Jersey, just across the Hudson River from Manhattan.

No nation or group had claimed responsibility, but analysts who specialize in terrorism immediately invoked the name of Osama bin Laden. The Saudi terrorist is believed to have orchestrated several attacks, including those on two U.S. embassies in Africa.

Mr. Bush said "the search is under way for those who are responsible for these evil attacks. We will make no distinction between the terrorists who committed those attacks and those who harbored them."

Air travel and stock trading were halted in the wake of the attacks. Military installations were put on high alert, and many government offices closed down, as did some private businesses.

Even in those workplaces that remained open, many employees huddled, wide-eyed, around television sets, unable to carry on business as usual.

In New York and Washington, officials hesitated to speculate about the number of fatalities. More than 50,000 people work in the 110-story towers, which collapsed after airliners slammed into their upper reaches Tuesday at 7:45 and 8:03 a.m. Dallas time.

The four airliners alone had 266 people aboard - more than were killed in the Oklahoma City bombing - and there were no known survivors. Based on cellphone calls from passengers and crew members on at least two of the jets, it appeared that the hijackers were armed with knives and box cutters. Some of the callers reported that the hijackers had stabbed one or more flight attendants.

Two of the planes were operated by American Airlines; the other two by United Airlines.

In New York, a firefighters union official said he feared an estimated 200 firefighters had died in rescue efforts at the trade center, and dozens of police officers were believed missing.

The onslaught was the worst attack waged against the United States since the Japanese bombing of Pearl Harbor in 1941.

"This is obviously an act of war that has been committed on the United States," said Sen. John McCain, R-Ariz.

Footage of death

New York Mayor Rudolph Giuliani said it probably would be Wednesday before informed estimates were available. What is certain, he said, is that the toll would be "more than any of us can bear."

At a nighttime news conference, he said rescuers have not given up on finding survivors. "Yes, there is hope that there are people still alive," he said.

It was 6 p.m. before fires abated enough for rescuers to enter the two-story pile of rubble that marked the site where the buildings had stood.

Mr. Giuliani said that 2,100 people were injured - 1,500 "walking wounded" who were taken to New Jersey, and 600 others who were taken to area hospitals, 150 of them in critical condition.

About an hour after the first New York attack, a third airliner crashed into the Pentagon in Washington, where 20,000 people work. Early estimates indicated that 100 people may have died there.

After nightfall, flames again poked through the roof of the massive military headquarters, which had been smoldering all day, and firetrucks kept up their steady stream trying to douse the fire.

Investigators

FBI analysts in Dallas on Tuesday pored over passenger flight logs provided by American Airlines in an effort to single out hijackers who were on board the passenger jets that destroyed the towers, said Special Agent in Charge Danny Defenbaugh.

Later, authorities said the FBI was preparing to search locations in Broward County in South Florida and Daytona Beach in central Florida. The locations had links to the suspected bin Laden supporter on one of the planes.

As recently as Friday, the State Department issued warnings of increased terrorist threats against U.S. citizens abroad. But there were no advisories about vigilance at home.

In June, a federal judge scheduled a bin Laden associate to be sentenced Wednesday at a courthouse near the Trade Center for his role in the bombing of the U.S. Embassy in Tanzania. However, that sentencing had been delayed some time ago.

In a development whose implications were unclear, explosions rocked the Afghan capital of Kabul late Tuesday afternoon. The United States has long accused the Taliban regime there of sheltering Mr. bin Laden.

Taliban officials denied Tuesday that Mr. bin Laden was behind the attacks on New York and Washington, and U.S. military spokesmen said the fireballs in Kabul were not the result of a U.S. strike.

They attributed the blasts to civil conflict within Afghanistan, leaving open the possibility that they were the work of American allies.

Evidence of planning

All the hijacked flights originated on the East Coast, giving the FAA and other officials little time to react once they were hijacked. And all were long-haul flights, full of fuel that essentially made them flying bombs.

Once they crashed, the spilled fuel would have spread flames throughout the stricken buildings, possibly melting their steel supports.

"This building would have stood had a plane ... smashed into it," said Hyman Brown, a University of Colorado civil engineering professor and the Trade Center's construction manager. "But steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."

Renee Cottrell-Brown of Arlington, vice president of Dallas-based Pro-Line Corp., was inside the second tower when the jet's impact rocked it. Before they fled, said her husband, Eric Brown, they saw people who had fallen or jumped from higher floors hurtling past their window.

Jennifer Riveiro, who works across the street from the Trade Center, said she had just emerged from the subway and noticed smoke billowing from one tower when the second one seemed to explode.

"It was like an inferno," she said. Her eyes burned as soot and debris rained down. She clung to the side of a building to avoid being trampled by people fleeing.

"It was a madhouse," Ms. Riveiro said.

"I don't know what to expect or what I'll be waking up to," she said.

Hundreds of volunteers and medical workers converged on triage centers, offering help and blood. Officials waited into the evening to launch a full-scale rescue operation, because fires still burned and other, nearby buildings had been weakened.

Their caution was vindicated around 4:20 p.m., when another, smaller building in the Trade Center complex collapsed under the weight of the debris.

Braced for the worst

He said the paramedics had been told that hundreds of police and firefighters, who had rushed to the scene in the immediate aftermath of the blasts, were missing.

In New York, where many subway lines shut down, thousand of workers streamed out of lower Manhattan on foot. By early afternoon, most of the streets were eerily quiet.

From her home in Brooklyn, Sheree Lane could see the spot where the towers had stood only hours before. "Now there are fighter jets circling over lower Manhattan," she reported. "That's something you don't want to see."

At the New York insurance agency where Diana Louros works, grief mixed with a sharper emotion. "Some people are very angry," Ms. Louros said. "But they don't know who to be angry at."