A More Immersive Documentary, Thanks to Google
HBO's Terror at the Mall goes light on presenting background info, on the assumption that viewers are going to look up the facts anyways.

If you’re watching TV, it’s probably not the only screen you’re looking at. Most viewers nowadays are online on some device while they watch, a growing trend that advertisers and networks are trying to take advantage of. Knowing that audiences have this “second screen” can also affect a director’s narrative choices.
“The way you watch television shapes the way you make a television program,” Reed told Quartz after a recent screening of the hour-long documentary. “And I know that when I watch a show now… if there’s something I don’t know, I will Google it.”
Indeed, in a 2013 survey of TV viewers worldwide, 75 percent multitask on mobile devices when they’re watching, and 49 percent said they use the Internet or an app to look up information about the content they’re watching.
TV Viewers Use Apps or the Internet to Look Up Facts While Watching

(Some of these numbers—Brazil, Chile, China, Russia, and Ireland—reflect only urban viewers.)
Relying on this “second screen” experience, Reed had the freedom to take his documentary style a step further. His approach has always been to focus in the personal stories that come from video footage and narratives of terrorist attacks—he has also made documentaries about the 2002 siege of a Moscow theater and the 2008 attack on a Mumbai hotel—but he said this documentary provides even less background and context than the those two, allowing him a sharper focus on the human stories of the day.
“I think it enables you to concentrate on what you can do on screen, rather than to have to provide a lot of information which would be conveyed through text,” Reed said. In the future, he said, he may provide contextual materials for viewers to glance at while watching a documentary.