SCOTT SIMON, HOST:

All morning, we've been hearing commemoration ceremonies to mark the moment that four flights crashed on September 11, 2001. In New York, ceremonies were held to mark - oh, forgive me - New York ceremonies were held to mark the moment the two flights hit the Twin Towers. 9:37, of course, American Airlines Flight 77 crashed into the Pentagon. And shortly after 10 a.m., United Airlines Flight 93, having been commandeered by passengers fighting with hijackers, crashed into a field in Shanksville, Pa. A tolling of the bells, moments and silence, followed by a reading of the names of the 2,977 people who perished that day. Transcript provided by NPR, Copyright NPR.