20 years after 9/11, mastermind Khalid Sheikh Mohammed still awaits trial. What went wrong?

Here's one perspective... 

It's all about how the justice system at Guantanamo was set up, says one national security expert: "The system is set up to fail."

https://www.nbcnews.com/politics/national-security/20-years-after-9-11-mastermind-khalid-sheikh-mohammed-still-n1278592

Here's another...