(NewsNation) — As the United States remembered 21 years since the terror attacks of Sept. 11 on Sunday, justice is still waiting to be served for five suspects who are considered architects behind the plan and are still being held at Guantanamo Bay in Cuba.

Now, potential plea deals could take the death penalty off the table for some of the suspects.

Khalid Shaikh Mohammed and four other men accused of 9/11-related crimes still sit in the U.S. detention center, their planned trials before a military tribunal repeatedly postponed.

The difficulty in holding a trial for Mohammed and other Guantanamo prisoners is partly rooted in what the U.S. did with him after his 2003 capture.

Mohammed and his co-defendants were initially held in secret prisons abroad. Hungry for information that might lead to the capture of other al-Qaida figures, CIA operatives subjected them to enhanced interrogation techniques that were tantamount to torture, human rights groups say. 

A Senate investigation later concluded the interrogations didn’t lead to any valuable intelligence. But it has sparked endless pretrial litigation over whether FBI reports on their statements can be used against them — a process not subject to speedy trial rules used in civilian courts.

The torture allegations led to concerns that the U.S. might have ruined its chance to put Mohammed on trial in a civilian court.

The Associated Press contributed to this report.