day after hijackers commandeered commercial jetliners and crashed them into two of the most visible symbols of America's financial and military might, President Bush once again condemned the attacks while investigators began piecing together leads and rescue workers continued their somber search through the ash-coated rubble.

The cloud of smoke and debris continued today to surround what had been two of the world's tallest buildings — a pair of quarter-mile-high landmarks at the World Trade Center that were as synonymous with New York as the Statue of Liberty or the Empire State Building. The two buildings collapsed on Tuesday after two Boeing 767's smashed into the 110-story towers. Also brought down was a wall of the Pentagon that was hit by an American Airlines Boeing 757.

With Vice President Dick Cheney at his left and Secretary of State Colin L. Powell at his right in the Cabinet Room at the White House this morning, President Bush called the attacks "more than acts of terror, they were acts of war."

Congressional officials said they planned to introduce a resolution condemning the attacks this afternoon.

"Freedom and democracy are under attack," the president said. "The American people need to know we are facing a different enemy than we have ever faced.

"This enemy hides in shadows and has no regard for human life. This is an enemy that preys on innocent and unsuspecting people and then runs for cover. But it won't be able to run for cover forever."

"We will rally the world," the president added. "We will be patient, we'll be focused and we'll be steadfast in our determination."

Also in Washington, Federal Aviation Administration officials prepared to allow air flights to resume sometime today, but they did not know when that would take place. A spokesman said passengers should not expect all flights to resume normal travel, since many planes are at the wrong airports.

In New York, Mayor Rudolph W. Giuliani said the death toll had risen to 45 and that "a few thousand people" were believed to be in the ruins of the World Trade Center. At the Pentagon, officials said that 80 bodies had been found and that there were no more survivors.

Mr. Giuliani said 1,000 to 2,000 rescue workers had worked through the night, searching for survivors. "The best estimate that we can make," the mayor said during a midmorning briefing, "is there were a few thousand left in each building," and that recovery and relief efforts were premised on "on those kinds of numbers."

He said that other survivors had been pulled from the wreckage, and that he was hopeful others would be found.

Governor George E. Pataki of New York said 18 search-and-rescue teams were working their way through the wreckage, and the city's police commissioner, Bernard B. Kerik, added that they were using listening devices in hopes of detecting sounds from survivors trapped in the thick layer of debris that coated lower Manhattan as the twin towers crumbled on Tuesday. The mayor, noting that it would take two to three weeks to clear the debris, said 128 dump trucks had been filled with rubble overnight.

The mayor said the police had received cell phone calls from people trapped in the debris. "There are people that are still alive," he said. "We'll be trying to recover as many people as possible and trying to clean up the horrible mess made by this."

As 750 National Guard troops arrived in New York, Mr. Pataki said that bridges and tunnels into Manhattan remained closed. The subways in New York were running this morning, but on some lines express trains were making all stops. And some crosstown buses were running on different routes.

Federal and city authorities reported a threat against the United Nations at about 9 a.m., the United Nations spokesman, Fred Eckhard, said. The United Nations ordered employees to leave its 39-story headquarters building or go to the basement, he said. But they returned to their desks after about 90 minutes, and the Security Council went ahead with its agenda, an open meeting at which the attacks are to be discussed.

At the Pentagon, thousands of Defense Department employees returned to their desks today in a building where the hallways still smelled of smoke and some offices lacked power. Military police officers with portable radios patrolled the corridors as firefighters continued to fight the blaze that began when the hijacked plane slammed into the 60-year-old building and burned through the night.

"They don't believe there's anyone alive," the news agency Agence France-Presse quoted a Pentagon official as saying. "No signs of life."

The building was ordered evacuated in midmorning. but the order was quickly rescinded. The Associated Press quoted Capt. Tim Taylor, a Pentagon spokesman, as saying, "It was a false alarm."

The Federal Bureau of Investigation set up a command center in Boston in its hunt for information about where the hijackers had come from. Governor Angus King of Maine said two men he described as suspects had flown to Boston from Portland, Me. They left behind a rental car that was impounded in the Portland area. Mr. King said the pair had apparently shown New Jersey drivers' licenses when they rented the car.

"This information appears to open up a series of leads that I'm sure will help to identify who the attackers," the governor told The Associated Press.

The Boston Globe said luggage belonging to one of the men did not make the Boston-to-New York flight that they hijacked. The Globe said the bag contained a copy of the Koran, an instructional video on flying commercial airliners and a fuel consumption calculator.

The Boston Herald said another rental car, containing Arabic-language flight-training manuals, had been found in a parking garage at Logan International Airport.

The Herald said the authorities were directed to a car by a traveler who called the Massachusetts State Police after hearing that the planes that sliced into the World Trade Center had taken off from Boston. The traveler said he had argued with the men as they parked the car on Tuesday.