david headley. >> david headley. >> narrator: david coleman headley, an american, recruited by pakistani terrorists to help plan the most spectacular terror attack since 9/11. >> so many innocent people lost their lives. >> narrator: but was his arrest really an intelligence success? >> nobody detected him, nobody stopped him, lots and lots of people died. >> narrator: propublica reporter sebastian rotella unearths new details about headley's shadowy life. >> it's a draft memoir. this is the first time it's been made public. >> narrator: and evidence of how high-tech spies accessed the terrorists' communications but still didn't stop the attack. >> what role did the program play in stopping headley? >> none. >> narrator: tonight frontline and propublica investigate an american terrorist. >> frontline is made possible by contributions to your pbs station from viewers like you. thank you. and by the corporation for public broadcasting. major support for frontline is provided by the john d. and catherine t. macarthur foundation, committed to building a more just, verdant and peaceful world. more infor