Lawrence Wright spent five years working on “The Looming Tower,” his Pulitzer prize-winning book about the lead-up to the 9/11 terrorist attacks. The book has been adapted into a drama series for the streaming service Hulu.

“My main sources were terrorists and intelligence agents,” Wright said Sunday at the Television Critics Association’s winter press tour. “They’re not the most open people to deal with. But honestly it was a mission for me. It’s probably the most important thing I’ll do in my professional life.”

Wright said that he “had to find a way to take this vast tragedy and make it human,” and his approach was to find key individuals and focus on their stories. One of those was individuals was John O’Neill, a counter-terrorism expert who became focused on Al Qaeda, was forced out of the FBI, and was hired as head of security at the World Trade Center — a job he began just weeks before the 9/11 attack in which O’Neill was killed.

“The man who was supposed to get bin Laden, bin Laden got him,” Wright said. “I used to think it was ironic. Now I think it’s Greek.”

Jeff Daniels plays O’Neill in the series.

“It’s a battle to get heard,” for O’Neill in the series, Daniels said. “The way that he delivered his message wasn’t always compatible with people listening to him. He was belligerent. He blasted people when he didn’t get his way. But in the end, he was right.”

Alex Gibney, who executive produces the series, compared working on the show to his experience as a documentary filmmaker.

“It’s inspired by true events,” Gibney said. “There are some key aspects of this central story that are absolutely vital to understand, and they’re essentially true. In the context of creating a drama that is compelling, inevitably characters are sometimes composited and stories rendered in dramatic terms. But in terms of understanding the roles that these agencies play and how they tracked Al Qaeda at the time and what their successes were and what their failures were, that’s pretty accurately presented.”

“The Looming Tower” is set to premiere on Hulu Feb. 28.