Attorneys for prisoners at the facility said at least one detainee claimed the documents were taken because prison officials suspected the lawyers might have had advance knowledge of suicide attempts, or even encouraged them as a form of protest - an allegation the lawyers deny.
"They think that they are going to find letters from us suggesting suicide. It's ludicrous," said Clive Stafford Smith, legal director for Reprieve, a British human rights group that has filed legal challenges on behalf of about 35 men held at the prison for terrorism suspects.
Bill Goodman, legal director of the Center for Constitutional Rights, which has filed challenges on behalf of about 200 detainees, said the group planned to have one of its attorneys look into the seizure and press for the return of papers during a visit to the jail next week.
"This is a huge breach of attorney-client privilege," he said.
US military officials declined to discuss whether papers were seized.
"They think that they are going to find letters from us suggesting suicide. It's ludicrous"
Clive Stafford Smith, legal director, Reprieve
Robert Durand, a spokesman at the Guantanamo base, referred questions about the legal papers to the Naval Criminal Investigative Service, which is handling the probe into the deaths, but also refused to comment.
The papers deal mostly with challenges filed on behalf of detainees in civilian courts in the United States.
On Thursday, the Supreme Court ruled the lawsuits could go forward even though Congress stripped detainees of the right to file the petitions in December 2005. The court ruled the law couldn't apply to legal challenges begun before it was enacted.
The court also ruled that President Bush's order to have military tribunals hold war crimes trials for detainees violated both US and international law. The Bush administration now is looking to Congress for authority to deal with suspected terrorists.
Washington is seeking Congress
help to try suspected terrorists
Lawyers said the military confiscated letters and other legal papers shortly after three detainees, two from Saudi Arabia and one from Yemen, hanged themselves inside their cells on June 10 and are the first deaths reported at the prison since it opened in January 2002.
Defence attorney, Richard Wilson, said in an affidavit that a military legal official told him that investigators had seized all personal papers from every detainee as part of the investigation.
Wilson, who represents a Canadian detainee, Omar Khadr, added in the affidavit that the official said he "did not believe that there is any investigation of attorneys themselves as to involvement or encouragement of the deaths."