WASHINGTON - Chilling radio transmissions by the Sept. 11 hijackers from the planes they commandeered were played for the first time Thursday, providing a vivid and horrifying portrait as they unfolded on that fateful day before confused air traffic controllers and military personnel. "We have some planes. Just stay quiet and you'll be O.K. We are returning to the airport," a hijacker, believed to be Mohamed Atta, the alleged ringleader of the 19 hijackers, told the passengers of American Airlines Flight 11. The tape was played for the audience at the commission's hearing. That transmission was the first inkling federal...