Bradley Manning, the source of the largest intelligence leak in US history, was allowed by his superiors to surf massive closed databases of secret information without any official restrictions, as well as download classified files to CDs and play music, movies and video games on his secure computer, his court martial has heard.

Day three of the trial, the highest-profile prosecution of an official leaker in at least a generation, focused on a tussle between the US government and Manning's defence lawyers over the environment in which the soldier worked as an intelligence analyst. The prosecution attempted to depict his unit within the 2nd Brigade Combat Team, 10th Mountain Division as meticulously trained in the handling and safeguarding of classified information.

By contrast, the defence team led by civilian lawyer David Coombs extracted answers from prosecution witnesses under cross-examination that presented the unit as an ill-disciplined group that operated under lapse security guidelines, even though they were stationed on active duty at a US military base outside Baghdad. Two of Manning's supervisors at Forward Operating Base Hammer were called to the stand, Jihrleah Showman and chief warrant officer Kyle Balonek, and grilled in similar fashion.

Showman was Manning's team leader for a brief period towards the beginning of their deployment to Iraq in November 2009 when they both did night shifts. Manning was assigned to intelligence gathering and analysis specifically on Shia-related insurgent groups.

Under cross-examination, Showman said that she was unable to recall any official stipulation of areas of the secure network of US official secrets – known as Siprnet – that intelligence analysts were barred from entering. There was no training provided on the restrictions of Siprnet.

Balonek, who supervised Manning after the soldier was moved to the daytime shift, added that the occupants of the secure area in which the intelligence analysts worked – a SCIF in military parlance – regularly listened to music, watched movies or played video games that had been downloaded to their secure computers. "If work was low it became allowed," Balonek said.

Asked by Coombs whether there were any restrictions on the members of the SCIF as to how much official intelligence they could download to CDs, Balonek replied: "Only the size of the CD, sir."

The judge presiding over the trial, who is sitting without a jury, Colonel Denise Lind, added her own question to Balonek that was clearly framed with Manning in mind. "If an intelligence analyst didn't want to watch a movie, and was interested in politics and wanted to search Siprnet, was he prohibited from doing that?"

"No, he was not," Balonek replied.

Manning faces 21 counts flowing from the transmission of hundreds of thousands of US classified documents to the open data website WikiLeaks. He has already pleaded guilty to leaking a collection that included US embassy cables, war logs from Iraq and Afghanistan, assessments of Guantánamo detainees and a video of an Apache helicopter attack on civilians in Baghdad.

The lesser offences to which he has admitted carry a maximum sentence of 20 years, but the government is pressing on with a slew of more serious charges headed by "aiding the enemy" that could put the soldier in military custody for the rest of his life.

The prosecution team led by Major Ashden Fein used its questioning to show how Manning, as a "35 Fox" intelligence analyst, was given repeated lessons about the importance of keeping classified information secret. Balonek read to the court excerpts of the non-disclosure agreement that Manning personally signed in 2008 in his presence.

The agreement said: "I understand that I have been granted access to classified information by the trust placed in me by the United States government. I have been advised that the unauthorised disclosure of classified information could cause damage to the US or could be used to the advantage of a foreign nation."

Fein asked Balonek whether Manning had been monitored every moment he spent in the Scif. "No," Balonek replied, "because he worked with us and had that level of trust. You have to trust the other analyst beside you, it's literally impossible to watch someone 24 hours a day and conduct your own analysis."

"If you were watching over PFC Manning and you saw him downloading an assessment to a CD then taking it to his [personal housing space] and put it on the internet, would you have stopped him?" Fein asked.

"Yes, sir," Balonek said.

"Is that something you noticed?"

"No, sir."

Earlier, Showman recalled conversations she had had with Manning while off-duty driving in her car and described his general interests. He liked to talk about politics, which he was passionate about, and government issues; she described him as being "on the extreme democratic side".

Showman said that "typically the conversations involved his topic of choosing. He talked about liking to attend martini parties in the DC area, having friends in politics and the Pentagon of influential status." He also liked shopping, she said.

At other times he indicated to her that he was "very fluent in anything computer – there was nothing he could not do on a computer". That included, she said, his ability to crack through passwords to portals within the classified computer networks.

Asked whether Manning ever talked about the internet, Showman said he once told her in her car that "he had to scrub the internet of everything about him, otherwise he would not be able to get a security clearance". The defence objected to the question, but Lind allowed it to stand in the record on the grounds that it spoke to Manning's "knowledge of the internet".

The point related to one of the most serious charges that the army private faces: violation of article 134 of the Uniform Code of Military Justice. Manning is accused of "wrongfully and wantonly causing to be published on the internet intelligence belonging to the US government, having knowledge that intelligence published on the internet is accessible to the enemy".