Using the wi-fi connection at Starbucks was a better bet than risking putting confidential defence documents on a glitch-prone Pentagon computer network, a senior US defence chief says.

The internet link at the local Starbucks was "the best bad option that we had," Air Force Colonel Karen Mayberry told the judge at the trial of five prisoners charged with plotting the September 11 hijacked plane attacks.

Defence lawyers have asked the judge to halt pre-trial hearings in the death penalty case of the alleged plotters at the Guantanamo Bay US naval base until the computer system can be fixed to ensure that outsiders cannot access confidential defence documents.

Col Mayberry ordered her team of lawyers to stop putting sensitive documents on that system in April, citing their ethical obligation to protect confidentiality.

The lawyers have since been using personal computers to email documents from coffee shops and hotel lobbies.

Col Mayberry said it was possible these networks were not secure, but she was certain that the Pentagon network had been compromised.

Col Mayberry cited evidence that defence files had been lost or altered, prosecutors and defence lawyers were temporarily given access to some of each other's emails, and outside monitors tracked defence researchers' work as they visited terrorism-related sites to prepare for the case.

"It's not speculative or hypothetical," Col Mayberry said. "It happened."

The network security debate has dominated the week-long hearing for the suspects, who could be executed if convicted of conspiring with al Qaeda, hijacking and murdering 2,976 people in the September 11, 2001, attacks.

Prosecution slams coffee shop security breach

Prosecutor Ed Ryan scoffed at the notion that using Starbucks wi-fi was safer than using the Pentagon network.

"You're not concerned about the nice man in the green apron looking over the major's shoulder as he's typing these emails?" Mr Ryan asked Col Mayberry.

Col Mayberry said that when she issued her April order she recognised that "it could shut us down" but that she thought the problems would be fixed quickly.

A logistics overseer testified that fixing the system could take up to 111 days once the Pentagon awards the contract and approves funding.

Some of the internet problems were blamed on a switch in email systems.

Internet technology supervisor Paul Scott Parr said a "dirty shutdown" occurred when the server shut down while a replication program was still running.

Backups that were supposed to occur daily had not been done for more than three months, Mr Parr said.

Seven gigabytes of data previously described as "lost" had merely been "misplaced" and had mostly been restored, he added.

Reuters