It's been 10 years since the attacks on the World Trade Center and the Pentagon, and still the questions linger -- including perhaps the ultimate question: Where was God on September 11th? In "Faith and Doubt at Ground Zero," FRONTLINE explores how the spiritual lives of both believers have been challenged since September 11 by difficult questions of good and evil, God's culpability, and the potential for darkness within religion itself. Through interviews with a cross section of Americans impacted by the attacks, FRONTLINE explores and illuminates the many spiritual questions that have come out of the terror, pain and destruction at Ground Zero. watch