Diane Bondareff / Associated PressPeople flee the scene near New York's World Trade Center after terrorists crashed two planes into the twin towers on Sept. 11, 2001.
Doug Mills / Associated PressPresident George W. Bush's chief of staff, Andy Card, whispers into the president's ear about the planes crashing into the World Trade Center as Bush was visiting Booker Elementary School in Sarasota, Fla., on Sept. 11, 2001.
Mario Tama / Getty ImagesA firefighter breaks down after the World Trade Center buildings collapsed after two hijacked airplanes slammed into the twin towers in a terrorist attack on Sept. 11, 2001.
Doug Kanter, AFP/Getty ImagesPedestrians can be seen crossing the Brooklyn Bridge as they flee Manhattan after the collapse of the first World Trade Center tower.
David Maxwell, AFP/GettyFirefighters and emergency personnel look through the debris of United Airlines Flight 93 after it crashed on the morning of Sept. 11, 2001 near Shanksville, Pa. 40 victims were killed in the crash after passengers reportedly stormed the cockpit to thwart terrorists' plans to use that plane as a weapon like the others.
Ernesto Mora / Associated PressTwo women hold each other as they watch the World Trade Center burn after a terrorist attack on the twin skyscrapers in New York on Sept. 11, 2001.
Spencer Platt / Getty ImagesA fiery blast rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building Sept. 11, 2001, in New York City.
Robert Mecea / NewsdayA firefighter screams in pain as he is rescued after being trapped under debris when one of the World Trade Center towers collapsed on Sept. 11, 2001.
Richard Drew / Associated PressA person jumps from the north tower of the World Trade Center in New York as another clings to the outside, left, while smoke and fire billow from the building on Sept. 11, 2001.
Marty Lederhandler / Associated PressHorrified bystanders in front of New York's St. Patrick's Cathedral look down Fifth Avenue toward the World Trade Center towers after planes crashed into their upper floors.
Steve Helber, APFirefighters hose down the twisted and smoldering remnants of one side of the Pentagon after American Airlines Flight 77 crashed into the Pentagon's west wall on the morning of Sept. 11.
Suzanne Plunkett / Associated PressPeople run from the collapse of the World Trade Center in New York on Sept. 11, 2001.
Stan Honda / AFP-Getty ImageA man covers his mouth to filter the air as he walks through debris after the collapse of one of New York's World Trade Center towers on Sept. 11, 2001.
Alex Fuchs, APThe remains of the World Trade Center stand amid smoke and debris after the terrorist attack on the New York buildings.
Spencer Platt / Getty ImagesThe twin towers of New York City's World Trade Center burn after two jetliners piloted by terrorists slammed into them on Sept. 11, 2001
Amy Sancetta / Associated PressThe south tower of the World Trade Center begins to collapse after a terrorist attack on the New York landmark on Sept. 11, 2001.
Graham Morrison, APA firefighter moves through piles of debris at the site of the World Trade Center.
Diane Bondareff / Associated Press
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People flee the scene near New York's World Trade Center after terrorists crashed two planes into the twin towers on Sept. 11, 2001.
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As traffic on the World Wide Web came to a crawl the morning of Sept. 11, 2001, chicagotribune.com communicated with many of its readers via e-mail. Here are the four Tribune Alerts that were sent that day to more than 19,800 readers — and that probably were forwarded to thousands of others.