Nine lawyers told the Washington Post they have written a Defense Department official to complain authorities are examining their clients' mail.
The nine include the attorney for Khalid Sheik Mohammed, the self-proclaimed mastermind of the U.S. terror attacks on Sept. 11, 2001.
Legal mail is the principal means of communication between detainees at Guantanamo Bay and their military defense attorneys in the Washington area, the Post reported.
A military official who spoke on condition of anonymity said the new commander at Guantanamo, Rear Adm. David B. Woods, has changed the mail policy.
Previously, military personnel opened the mail in the presence of detainees to ensure there was no contraband and then handed it to them without reading the contents.
Last month Woods changed the policy and insisted on establishing that all communications were relevant to the cases, the official said.
A Pentagon spokesman declined to comment, saying the matter was the subject of ongoing litigation.