John P. O’Neill was only weeks into his new job as head of security at the World Trade Center. Before that, he had spent decades with the FBI, becoming one of its leading counterterrorism experts and repeatedly warning about the growing threat from al-Qaeda. When Flight 11 struck the North Tower, O’Neill was in the South Tower. Instead of leaving the complex, he began assessing the damage and helping coordinate the response. He was last seen heading back toward the South Tower as the emergency worsened. O’Neill died when the towers collapsed. He was 49 years old. The man who had spent years trying to understand al-Qaeda ultimately lost his life helping others during the attack he had long feared.