* Lawyers say legal files began disappearing in February
* Judge postpones hearing in USS Cole case
* Lawyers in Sept. 11 case also want a delay (Updates with detail, quotes, background)
By Jane Sutton
MIAMI, April 11 (Reuters) - Pretrial hearings in the Guantanamo war crimes tribunals have been delayed to address the mysterious disappearance of defense legal documents from Pentagon computers, military officials said on Thursday.
The defense lawyers said their confidential work documents began vanishing from Pentagon computers in February and that there was evidence their internal emails and internet searches had been monitored by third parties.
They want all the hearings in both death penalty cases halted until the issues have been satisfactorily addressed.
A weeklong hearing was scheduled to start on Monday in the case of Abd al Rahim al Nashiri, who is charged with masterminding an attack that killed 17 U.S. sailors aboard the USS Cole off Yemen in 2000.
That has been pushed back to June 11, the judge overseeing the war crimes court at the Guantanamo Bay U.S. Naval Base said in an order on Thursday.
Defense lawyers said they also would ask the judge, Army Colonel James Pohl, to delay a week of pretrial hearings set to begin on April 22 for five prisoners charged with plotting the Sept. 11 hijacked plane attacks.
Navy Commander Walter Ruiz, who represents 9/11 defendant Mustafa al Hawsawi, said "three to four weeks' worth of work is gone, vanished."
He said what appeared to be a computer folder of prosecution files had turned up on the defense lawyers' system, though none of them had opened the files.
The chief defense counsel for the tribunals, Colonel Karen Mayberry, ordered military and civilian defense lawyers on Wednesday night to stop using their government computers for sensitive information or drafts of their work.
"I'll be filing a hand-written motion very shortly to ask for an abatement of the proceedings," in the 9/11 case, said defense attorney James Connell, who represents defendant Ali Abdul Aziz Ali.
In another case, system administrators were searching files at prosecutors' request and were able to access more than 500,000 defense files, including confidential attorney-client communications, the lawyers said.
That incident involved an appeal on behalf of Ibrahim al Qosi, a Sudanese prisoner who had finished his sentence at Guantanamo and gone home, they said.
Defense lawyers said their files began disappearing after a February hearing during which intelligence agents outside the courtroom cut the closed-circuit feed that was broadcasting the proceedings to spectators and journalists. The judge ordered technicians to dismantle the system that allowed them to do that.
During that hearing, the Guantanamo detention camp's legal advisor also disclosed that what appeared to be smoke alarms in the rooms where defense lawyers met with their clients were actually microphones. He said private attorney-clients conversations had not been monitored, a claim met with skepticism by defense lawyers.
Human Rights First, a longtime critic of the Guantanamo tribunals, called the latest disclosures "absolutely outrageous."
"This is just further evidence that the military commission system is a sham and that all terrorism trials should be held in real U.S. federal courts on U.S. soil, where the rules are clear, defendants' rights are respected and the verdicts will have credibility," said Daphne Eviatar, who has monitored the tribunals for the rights group. (Reporting by Jane Sutton; Editing by David Adams and Vicki Allen)