Watch: Third and fourth moment of silence in NYC and timeline of 9/11 attacks

At 9:59 a.m. ET on Sept. 11, 2001, the South Tower collapsed at the World Trade Center in New York City during a series of terrorist attacks that changed the U.S. forever. At 10:03 a.m. ET, the passengers on United Airlines Flight 93 launched a counterattack on the hijackers aboard their plane, causing a crash into a field near Shanksville, Pennsylvania. It's believed the aircraft may have been headed toward Washington, D.C.