If there is any building that symbolizes American national security, it is the Pentagon - the massive, five-sided office complex that is the headquarters of the U.S. Defense Department.

But that symbol was severely tarnished Tuesday as an airliner, apparently commandeered by terrorists, slammed into the building cutting a path of destruction through a portion of the five-story structure. One Pentagon spokesman tells VOA he believes there were an extensive number of casualties.

Senior Pentagon officials, including Defense Secretary Donald Rumsfeld, are reported to be safe. The estimated 20,000 military and civilian workers at the building were evacuated.

U.S. military forces worldwide have been placed on a heightened state of alert in the wake of the attack, which occurred after earlier apparent terrorist strikes against the twin towers of the World Trade Center in New York City.

Eyewitnesses say the plane that struck the Pentagon was a commercial airliner operated by American Airlines. They say it came in low over a roadway near the building, clipping light poles.

Inside the Pentagon, witnesses reported being thrown to the floor by the force of the explosion that occurred when the plane struck.

It was not immediately clear when the building would reopen.

Smoke was still billowing out of the collapsed portion of the structure hours after the crash as emergency crews struggled to control fires.

There are extensive security measures in force at the Pentagon, designed to control ground access into the building. However there were apparently no measures designed to prevent against an airborne attack. The Pentagon is located close to one of Washington's major airports and planes, both landing and taking off, are often routed along a flight path that runs virtually over the building.