The September 11 attacks had a profound impact on the world as we know it. Global security and foreign relations have been greatly affected ever since the four coordinated terrorist attacks were carried out on American soil. Changes in airport security, such as stricter TSA regulations, have become a permanent part of air travel, while unrelated fields, like medical research and domestic policy, have also felt the negative effects over the decades.
This event has been researched and documented from seemingly every possible angle for the past 20-plus years, and yet there are still some parts of the tragedy that remain unknown or debated. Intrigued? Click through this gallery to discover the questions about that fateful day that remain unanswered.