Two senators watched confessed Sept. 11 mastermind Khalid Sheikh Mohammed claim responsibility for 31 terrorist attacks and plots while at the U.S. detention facility at Guantanamo Bay, Cuba.
Graham described the experience as "bone chilling," saying Mohammed appeared to be in good health with a long beard.
"It was apparent that KSM views himself as a warrior, motivated by religious teachings, and seeks his place in history," according to a joint statement released Friday by Graham and Levin.
Mohammed's confession was read by a member of the U.S. military who is serving as his personal representative, outlining his involvement in the Sept. 11, 2001, attacks, personally beheading Wall Street Journal reporter Daniel Pearl and other terrorist activities.
"I was responsible for the 9/11 operation from A to Z," Mohammed said in a statement read Saturday during a Combatant Status Review Tribunal at the U.S. detention facility at Guantanamo Bay, Cuba.
"We were impressed with the professionalism and demeanor of the tribunal. KSM was given the opportunity to speak freely. He did so in a non-disruptive way and was respectful to the tribunal personnel," according to Graham and Levin's statement.
A personal representative for Mohammed, who was not a lawyer, read a statement detailing preparation for the Sept. 11, 2001, terrorist attacks, involvement in executions and other terrorist activities. Mohammed replied in English that he understood the statement. The interpreter was used rarely.
Mohammed also submitted a written statement alleging mistreatment during his captivity before Guantanamo Bay. Mohammed's allegations are under view by officials.
"Allegations of prisoner mistreatment must be taken seriously and properly investigated," according to a joint statement from Graham and Levin. "To do otherwise would reflect poorly on our nation."
FOX News' Trish Turner contributed to this report.