WASHINGTON - U.S. investigators now believe that a hijacker in the cockpit aboard United Airlines Flight 93 instructed terrorist-pilot Ziad Jarrah to crash the jetliner into a Pennsylvania field because of a passenger uprising in the cabin. This theory, based on the government's analysis of cockpit recordings, discounts the popular perception of insurgent passengers grappling with terrorists to seize the plane's controls. The government's findings — laid out deep within the report on the Sept. 11, 2001, attacks that was sent to Congress last month — aim to resolve one of the enduring mysteries of the deadliest terror attacks in...