Attorneys for Army intelligence analyst Bradley Manning on Monday challenged evidence linking him to the biggest classified document leak in U.S. history, arguing others had access to the same files and that it cannot be proven Manning sent anything to WikiLeaks from his computer.

The 24-year-old Manning is suspected of downloading thousands of classified or confidential documents from the military's Secret Internet Protocol Router Network, or SIPRNet. Those files are thought to have later appeared on the whistleblower website WikiLeaks.

Manning is currently facing a military hearing to determine whether there is sufficient evidence to court martial him on 22 charges including aiding the enemy and unlawfully causing intelligence to be published on the Internet. He faces life imprisonment if convicted of the most serious charge.

Government prosecutors have portrayed Manning as a well-trained soldier adept at using computers who violated his duties. Manning's defense lawyers have underscored evidence that he was an emotionally troubled young man and have questioned why he was not relieved of his intelligence duties sooner.

Witnesses have said Manning sent an email to his sergeant saying confusion over his gender identity was seriously hurting his life, work and ability to think. Manning had created a female alter-ego online, Breanna Manning, according to testimony at the hearing.

A military computer crimes investigator, Special Agent David Shaver, testified on Sunday that he found files on Manning's computer that mirrored those that appeared on WikiLeaks, the first time the government has offered direct links between Manning and the anti-secrecy website.

Shaver said he recreated a path used on Manning's computer to download assessment documents about detainees in the U.S. war against al Qaeda who are being held at the U.S. military prison at Guantanamo Bay, Cuba.

Four complete Guantanamo detainee assessments were among files that had been deleted from Manning's user profile, Shaver testified. Similar documents later showed up on the WikiLeaks website.

Shaver also testified he found thousands of U.S. State Department diplomatic cables that had been downloaded to Manning's computer. Some 10,000 were in a corrupted folder, while 100,000 others were in a special format for easy transfer.

Under cross-examination on Monday by Captain Paul Bouchard, a member of Manning's defense team, Shaver said he compared some of the cables in the corrupted folder on Manning's machine with those on WikiLeaks and discovered none of them matched.

Witnesses have also testified that analysts were encouraged to look at diplomatic cables to get a big-picture view of the war in Iraq, where Manning's unit was stationed.

Shaver acknowledged that no forensic evidence could prove any of the information that appeared on WikiLeaks had been sent from Manning's computer, or even that Manning had been using the computer when the files were downloaded.

I could not put somebody at the keyboard, Shaver said.

Defense lawyers argued that other analysts on Manning's team had access to the cables, which were not password protected and could be easily downloaded, witnesses said.

Shaver testified he found an unauthorized program on Manning's computer that enabled users to swiftly download large numbers of files. But witnesses acknowledged the program was a helpful data-mining tool that may have been used by several of the analysts as part of their work.

Investigators found Manning's computer had two versions of an Apache helicopter gunsight video, one of which appeared to be the one aired by WikiLeaks and another that may have been the source material, Shaver said. The video showed an attack that killed several Iraqis, including two Reuters journalists.

Bouchard pointed out that analysts in Manning's unit had discussed the video in December 2009 and had watched it on several computers.

(Writing by David Alexander)

(Editing by Todd Eastham)