8:45 a.m.: A hijacked passenger plane flew into 1 World Trade Center (WTC), North Tower, in New York City. Its full tanks of jet fuel set the upper floors on fire. Tens of thousands of people in the towers began to evacuate in an orderly manner, the way they had been taught in their fire drills. Thousands of people from the surrounding buildings also started to leave the area, mostly on foot.

9:03 a.m.: A second plane crashed into 2 WTC, South Tower, setting its upper floors on fire. People continued to evacuate. Hundreds of firefighters arrived on the scene to help.

9:45 a.m.: Another hijacked plane crashed into the Pentagon, in Washington, D.C., starting a fire. Those Pentagon workers not in secure underground areas were evacuated.

9:50 a.m.: The South Tower collapsed, killing most of those left inside.

10:15 a.m.: Another hijacked plane crashed in a field in southwestern Pennsylvania. Based on cell phone calls made from the plane, we know that some passengers heard of the attacks in New York City and bravely decided that if they were going to die, they would die immediately rather than allow hundreds of others to be murdered by their plane.

10:30 a.m.: The North Tower collapsed, killing most of those left inside. (Note: Over 30 years ago the architect of the WTC had to figure out what should happen if either or both of the Towers were damaged too badly to stand. The architect designed each Tower to collapse on itself, like a telescope. Had either Tower toppled sideways like a tree onto neighboring streets and buildings, thousands of people who had made it to safety would have been killed. It's comforting to know that someone's calm foresight all those years ago saved thousands of lives on September 11.)