Military and civilian employees of the Pentagon filed to their jobs even as smoke from the stricken section of the building wafted over nearby Virginia hillsides and highways. Teams that worked overnight to shore up walls heard no sound of survivors but did not rule out the possibility of finding some trapped alive once they were able to get into the rubble with cameras and microphones.

In a day-after scare, thousands of employees at the Agriculture Department's main building along the National Mall were sent home. Reports of an unidentified aircraft in Canadian airspace prompted the evacuation, Chris Gomez, deputy director of the department's office of operations, told employees. Federal Aviation Administration spokesman Les Dorr said his agency was unaware of any such plane.

President Bush was in the Oval Office at 7 a.m., meeting his national security adviser, and spoke by phone with prime ministers Tony Blair of Britain and Jean Chretien of Canada.

''They all expressed their resolved to stand strong against terrorism,'' Bush spokesman Ari Fleischer said. Bush thanked Chretien for Canada's help diverting and landing American planes in Tuesday's chaos.

Powell reinforced Bush's assertion that the deadly attacks would be avenged.

''It's a war not just against the United States, it's a war against civilization,'' he said.

Powell, making the rounds of morning talk shows, said the administration was ''far from selecting any particular military targets'' for retaliation against the attacks that brought down the towers of the World Trade Center in New York and four hijacked U.S. jetliners and breached the Pentagon.

''I would not remove any of the options available to the president,'' he said. But ''we have to build the case first.''

Powell said Americans have made the judgment: ''We are at war and they want a comprehensive response. They want us to act as if we are at war and we're going to do that - diplomatically and militarily.''

The State Department, which closed after the attacks, opened again. Powell said one-quarter of U.S. embassies were still closed as a precaution.

The U.S. air space, closed to commercial traffic for the first time, was expected to reopen at noon EDT, said Dorr, speaking for the FAA. He said it would take time for airlines to get back to normal schedules, with so many planes diverted to wrong locations.

He suggested passengers who would normally go the airport an hour before a flight should go two hours in advance.

At the White House, Bush and his wife, Laura, were asking Americans to donate blood, spokesman Gordon Johndroe said. Bush was also meeting his national security team.

Pentagon workers were able to enter sections of their building but nearly half the structure had no power and some employees were asked not to show up. Among those at their desks were Defense Secretary Donald H. Rumsfeld and Army Gen. Henry H. Shelton, chairman of the Joint Chiefs.

Bush, addressing the nation Tuesday night, condemned the ''acts of mass murder.'' The death toll in New York was unknown but thought to be surely in the thousands; the Arlington County, Va., fire department estimated 100 to 800 people died in the Pentagon attack.

''Our military is powerful, and it's prepared,'' a somber Bush said in his Oval Office address. Administration officials said early evidence pointed toward suspected terrorist Osama bin Laden, harbored in Afghanistan.

Bush said in his televised address, ''We will make no distinction between the terrorists who committed these acts and those who harbor them.''

Streets around the White House were reopened overnight, but the capital city remained under close watch as Wednesday dawned with more police on patrol than usual. They were supplemented by 30 to 50 National Guardsmen stationed - in pairs with a Humvee - at more than a dozen street corners in the main business section.

Powell said he knew of no indications that terrorists would immediately follow up on their assault. There was ''nothing to suggest that there is something waiting to happen today.''

And he said it was realistic to expect that Americans could track down the terrorists. ''Sometimes it takes a few weeks, sometimes it takes years. But we won't give up. We will find them and they will be dealt with.''

Lawmakers also arranged to convene to condemn the terrorism - a day after the Capitol was evacuated and congressional leaders were hastily ferried to an underground bunker 75 miles away.

''The Pentagon is functioning,'' a defiant Rumsfeld said Tuesday night, despite the crash that sent a bright orange fireball skyward, caused the collapse of a section of one of the massive building's five sides.

Downtown federal agencies also were reopened.

Administration sources 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 a suspected bin Laden supporter whose name was on the manifest of one of the hijacked jetliners.

Sen. Orrin Hatch, R-Utah, said U.S. intelligence intercepted communications between bin Laden supporters discussing the attacks.

The suspected fugitive terrorist has been sheltered in Afghanistan, but that nation's hardline Taliban rulers rejected suggestions he was to blame.

''I have no intent of discussing today what comes next,'' Shelton said. ''But make no mistake about it, your armed forces are ready.''

Officials declined repeatedly Tuesday night to estimate the number of injured or dead in the attacks. Bush himself referred to ''thousands of lives'' being ended and Sen. Hillary Clinton, D-N.Y., said ''I don't think we've had an inkling of the devastation'' in downtown Manhattan.

Amid the devastation, Bush offered reassuring words. ''These acts of mass murder were intended to frighten our nation into chaos and retreat. But they have failed,'' he said. Our country is strong. A great people has been moved to defend a great nation.''

House Speaker Dennis Hastert seconded that in remarks on the Capitol steps at twilight. ''We will stand together to make sure that those who have brought forth this evil deed will pay the price,'' he said. Moments later, he and scores of members of the House and Senate raised their voices in a rendition of ''God Bless America.''

The well-choreographed displays of public confidence-building seemed unlikely earlier in the day.

Bush awoke in Florida, and when word came of the attacks he was flown first to a secure military base in Louisiana, then to an underground facility in Nebraska where he convened a meeting of the National Security Council by telephone

Vice President Dick Cheney remained behind in Washington in a secure region of the White House. But Hastert, next in line for the presidency behind Cheney, was taken first to Andrews Air Force base outside Washington, then flown 75 miles to a government bunker. Other congressional leaders joined him there. And according to one person in attendance, they were able to monitor events on two televisions, talk occasionally with Cheney and receive a briefing from administration officials.

The Pentagon crash sent billows of smoke over the Potomac River toward the nation's capital. Authorities said there were more than 60 passengers and crew aboard the plane, an American Airlines jet that had taken off moments earlier from Washington Dulles International Airport bound for Los Angeles.

Lawmakers briefed by law enforcement officials late Tuesday said knives seemed to be the weapons used by the hijackers in three of the four planes, based on cell phone calls.

Rep. Allen Boyd, D-Fla., said law enforcement officials estimated three to five terrorists were on each plane.

The fourth hijacked plane crashed in rural Pennsylvania about 80 miles southeast of Pittsburgh, after making a U-turn toward Washington.