The project began when Ali Soufan and Lawrence Wright realized they were both being separately approached by studios for the rights to their respective books about the lead-up to 9/11, prompting them to team up.

Soufan and Wright brought on documentary filmmaker Alex Gibney, who had already adapted Wright's 'My Trip to Al-Qaeda' and 'Going Clear' for HBO, because they trusted his ability to handle difficult moral questions with a documentarian's regard for truth.

Although the producers pitched the series to HBO, Netflix and Amazon and only approached Hulu as a courtesy, Hulu ultimately won the project by offering a straight-to-series order, a faster development process, a promise not to bow to pressure from the federal agencies depicted, and the largest financial commitment of any platform they met with.

Beyond adapting Lawrence Wright's book, the writers also drew material from Ali Soufan's own book 'The Black Banners' and from the official 9/11 Commission Report.

The character Diane Marsh, a CIA analyst played by Wrenn Schmidt, is a composite figure based on at least three real people within the CIA, including Alfreda Frances Bikowsky.

Bill Camp's character Robert Chesney is described as the most composited character in the series, with Ali Soufan himself noting he combines traits from at least four real FBI figures.

To prepare for playing the late John O'Neill, Jeff Daniels spent an evening at a Lower Manhattan bar with ten of O'Neill's real former colleagues, an experience he said helped him connect with the spirit of what O'Neill was fighting for.

Filming lasted six months and took place across three continents and six countries, with Johannesburg, South Africa serving as the main production base standing in for locations including Nairobi, Tanzania, England, Albania, and Las Vegas.

Scenes set in Yemen were actually filmed in Morocco, while drone and car travel shots meant to depict Pakistan were used to evoke Afghanistan.

To recreate the 1998 U.S. embassy bombing site in Nairobi, the production built a large rubble pile in Cape Town's central business district that shut the area down for weeks, informed by research including a set dresser's personal photos from his tour in Afghanistan and archival newspaper research conducted in Nairobi.