The US soldier accused of leaking thousands of government secrets struggled with emotional problems and gender issues, a court has heard.
Private Bradley Manning faces 22 charges of distributing state secrets to anti-secrecy site Wikileaks.
But his defence have questioned whether he should have been given access to the sensitive documents in the first place.
The hearing, at Fort Meade army base in Maryland, will determine whether Pte Manning should face a court martial.
The hearing began on Friday under tight security and is expected to last around five days.
The defence have failed to get Lt Col Paul Almanza dismissed as presiding officer for the prosecution.
They had said Lt Col Almanza's refusal to accept all but two of 38 defence witnesses meant the defence could not adequately make their case, but the army's appeals court rejected their concerns.
Lt Col Almanza, is a former military judge who now works for the Department of Justice, which has its own investigation into Wikileaks.
Under cross-examination of military investigators, details emerged of incidents during Pte Manning's deployment as an intelligence analyst in Iraq between November 2009 and his arrest in May 2010.
His defence lawyer, David Coombs, highlighted emails his client had sent to a superior officer explaining that confusion about his gender identity was impacting on his ability to do his job.
Investigators admitted they had found evidence that Pte Manning had created an online alter ego called "Breanna Manning".
The soldier had also reportedly assaulted a superior, turned over a table, damaged a computer and on another occasion was found "curled up in a ball".
Mr Coombs pointed out that hundreds of thousands of government employees had access to the military's classified network, and yet Pte Manning's access was never revoked.
"If you were told of this behaviour would you consider it a minor incident?" Mr Coombs asked Capt Steven Lim who was Pte Manning's officer in charge in Iraq.
"Probably not," Capt Lim answered.
The prosecution began making their case during Saturday's session.
Pte Manning is accused of the unauthorised possession and distribution of more than 720,000 secret diplomatic and military documents.
He has also been charged with "aiding the enemy", a charge that could carry the death penalty although prosecutors are reportedly only seeking a life prison sentence if he is convicted.
Don't ask, don't tell
The defence says Pte Manning was suffering from stress, as a homosexual soldier at a time when soldiers in the US military could not be open about their sexuality - under the "don't ask, don't tell" policy, which has since been repealed by US President Barack Obama.
His defence team is also expected to argue that little harm came of the leaks, and that their release was in the greater public interest.
Outside the army base, more than 100 people demonstrated in support of Pte Manning on Saturday.
Some held signs reading "Americans have the right to know. Free Bradley Manning" and "Blowing the whistle on war crimes is not a crime", AP news agency reports.