In a letter obtained by the American Civil Liberties Union (ACLU), the 9/11 Commission was refused permission to question terror suspects, with the Bush administration arguing that by doing so the panel would "cross" a "line" and obstruct the administration's efforts to protect the nation.
Citing the need to "Safeguard the national security, including protection of Americans from future terrorist attacks," the government officials demanded the Commission not make further attempts at conducting a deeper probe into the September 11 terror attacks.
"As the officers of the United States responsible for the law enforcement, defense and intelligence functions of the Government, we urge your Commission not to further pursue the proposed request to participate in the questioning of detainees," the letter said.
The warning note dated January 6, 2004 has been signed by former Attorney General John Ashcroft, Defense Secretary Donald Rumsfeld and CIA Director George Tenet.
The 9/11 Commission was set up in November 2002 to prepare a full account of the circumstances surrounding the attacks, which still have an air of suspicion around them.
The revelation comes as according to a recent survey, one-quarter of US adults believe that the 2001 terrorist attacks were fabricated.