Attorneys for US Army Private Bradley Manning begin Wednesday presenting their defense of the low-ranking soldier blamed for one of the biggest intelligence breaches in US history.

The defense is to call its first witnesses to the stand at a hearing being held on this sprawling army base to determine whether Manning should face a court-martial on charges that could send him to prison for life.

The government rested its case Tuesday after calling a parade of witnesses that included soldiers who served alongside Manning, digital forensics experts who examined his computers and the man who reported him to the US authorities.

Manning, 24, is accused of giving secret-spilling site WikiLeaks a trove of US military reports from Iraq and Afghanistan, classified State Department cables, Guantanamo detainee assessments and videos of US air strikes.

US Army investigators have testified that contact information for Julian Assange, online chats with the WikiLeaks founder, and a trail of incriminating digital footprints was found on Manning's computers and other devices.

Some of the most dramatic testimony came on Tuesday as Adrian Lamo, the former computer hacker who turned Manning over to the authorities, told the court how the junior intelligence analyst reached out to him.

Dressed in a green camouflage uniform of the 10th Mountain Division with an American flag patch on his sleeve, Manning alternated between looking intently at Lamo through black-rimmed glasses and jotting down notes on a legal pad.

Lamo, a figure of some renown among hackers around the world, recounted how an individual who he said eventually turned out to be Manning had reached out to him by email around May 20, 2010.

Lamo said he began holding encrypted AOL instant message chats with someone using the screen name "bradass87" and he later confirmed it was Manning after receiving a Facebook friend request from him.

In the online conversations, Manning talked about supplying classified US material to a "crazy white-haired Aussie," an apparent reference to Assange.

"I want people to see the truth, regardless of who they are, because without information you cannot make informed decisions as a public," Manning said in the chat logs published by Wired.com.

Such statements have made Manning a hero to anti-war activists and supporters, including Pentagon Papers whistle blower Daniel Ellsberg, have attended his hearing and held vigils and rallies outside of Fort Meade.

The US government has denounced the document dump as a "criminal" act which endangered confidential sources, national security and foreign policy.

Asked why he contacted the authorities, Lamo said: "What I saw in those chats was an admission of acts so egregious that it required that response."

Lamo was also asked why he thought "bradass87" had contacted him. "I do not believe they were looking for guidance so much as I think they were looking to brag about what they had done," he said.

The court also heard Tuesday from a former supervisor of Manning, ex-army specialist Jihrleah Showman, who said she had recommended that he not deploy to Iraq with the rest of the unit because of mental health and behavioral issues.

The defense has suggested that Manning struggled with gender issues and emotional problems but his superiors repeatedly failed to provide counseling, take disciplinary action or revoke his security clearance.

They have also indicated that Manning, who is gay, had difficulty serving in a military that was operating under the "Don't Ask, Don't Tell" policy towards homosexuals, which since has been repealed.

Following the defense presentation, which is expected to last about a day, the investigating officer may take several weeks before deciding whether or not to proceed with a court-martial.

Manning is facing 22 charges, the most serious being aiding the enemy, which carries a life sentence. Aiding the enemy can be a capital offense but the military has said it will not seek the death penalty.

Assange, under house arrest in Britain awaiting potential extradition to Sweden to face sexual assault charges, has denied knowing the source of the leaks, but has expressed support for Manning and raised funds for his defense.