UPDATED AT 9:43 AM  EDT
Striking at the heart of U.S. military power, an airliner, apparently steered by hijackers, plowed into the Pentagon causing extensive damage and pushing the number of victims from Tuesday's attack even higher.

When the jet collided with the structure, one of the five walls collapsed then burst into flames. For most of the day, smoke billowed up from the wreckage, and emergency teams raced to pry the injured and dead from the twisted metal and concrete.

"The whole building shook," said Terry Yongers, an air force civilian who was in the building at the time of the strike. "There was screaming and pandemonium," he said.

At a news conference, Defence Department secretary Donald Rumsfeld said the evacuation of 20,000 employees was carried out smoothly and professionally.

"The leadership of the Defence Department is O.K," Flood told reporters, adding that the departments of Justice, State, Treasury and Defence and CIA empolyee were sent home until they're notified.

The Capitol building was also evacuated, and lawmakers and their aides have been told that the House and Summit may reconvene later this week.

Glenn Flood, a Pentagon spokesman, said there were "extensive casualties and an unknown number of fatalities."

U.S. Federal Reserve Chairman Alan Greenspan was not at the Fed's Washington building at the time of the attack, a Fed spokesman confirmed. The Secretary of the Defense Department, Donald H. Rumsfeld, is also okay, according to a U.S. official.

Away from the Pentagon, a number of unexplained explosions were reported close to the State Department and the Capitol. Reports also suggest a second hijacked passenger plane headed for the Pentagon crashed near Camp David.

While it is not known if it was shot down, F-16 fighter jets were headed in the planes direction.

"This is the second Pearl Harbor. I don't think that I overstate it," said Sen. Chuck Hagel, R-Neb., referring to the attack 60 years ago that surprised the nation and propelled it into the Second World War.