Five minutes before Flight 93 crashed in Pennsylvania, the South Tower, (which was hit by the second plane) collapsed at 10:05 EDT. Hundreds of rescue workers, plus people trying to escape, were crushed to death.

Twenty-four minutes later, the North Tower also collapsed. A cloud of volcanic-like ash rushed through the streets of lower Manhattan as people fled for their lives. It was too late, however, for rescuers and people inside the building. They, like so many others in the South Tower, died on September 11.

Some people survived. They survived because of miracles or heroes. Heroes like Rick Rescorla who made sure nearly all of Morgan Stanley’s 2700 employees left the South Tower even while building announcements advised people to stay put.

Rescorla was never one to follow advice he didn’t trust. He knew what danger was - he had faced it in Vietnam. Because of him, thousands of people were not victims - as he was - of the terrorist plot.

The WTC complex was damaged. At 5:20 p.m. EDT, Number 7 World Trade Center (a 47-story building) also collapsed. At the time, although it was not well-publicized, officials feared that the Hudson River could flood the entire area if the WTC foundations caved in.

But what, exactly, caused the towers to fall?