US Army Private Bradley Manning's defense team sought Monday to cast doubt on some of the charges he faces for passing hundreds of thousands of secret government files to WikiLeaks.

At the start of the second week of Manning's trial, the court heard that one software program he is said to have illicitly added to his army computer while stationed in Iraq was used by everyone in the intelligence cell where he worked.

The government's leading computer forensics expert in the case also conceded that a second program used by Manning to download hundreds of documents had not been outlawed by military commanders.

Manning has admitted handing diplomatic cables and battlefield reports from the wars in Iraq and Afghanistan to WikiLeaks but he denies "aiding the enemy," chiefly Al-Qaeda, a charge that could see him jailed for life if convicted.

Chad Madaras, who worked alongside Manning at Forward Operating Base Hammer in eastern Baghdad, sharing the same computer on alternating shifts, testified that an Internet chat program, known as mIRC, was widely and openly used.

"Everybody had it" on their computers, Madaras, who held the rank of sergeant when he left the army in January this year, said at Manning's court-martial at Fort Meade military base in Maryland.

Asked by Manning's civilian defense lawyer if the program had been banned, Madaras replied: "No, sir."

The slew of charges that Manning faces, 21 in total, includes counts that he breached army regulations by using unauthorized software for unintended purposes and that he circumvented security systems and acted beyond his authority.

WikiLeaks's decision to publish some of the material it received from Manning caused deep embarrassment to US political leaders and led to his arrest in Iraq in May 2010. He has been held in military detention ever since.

The government argued in the first week of the trial that Manning, 25, set out to leak information almost from the moment he arrived in Iraq in November 2009. The defense has said Manning acted months later.

And the defense pressed further on Monday, seeking to undermine the charges that Manning had used a second piece of unapproved software.

Special Agent David Shaver, who led the electronic audit of Manning's computer, admitted under cross examination that WGet -- a program that Manning used to download hundreds of Guantanamo detainee reports -- was not forbidden.

Shaver, of the army's Computer Crimes Investigative Unit, then said Manning probably first searched the Internet in early December 2009 -- two weeks into his deployment -- for information on WikiLeaks and its founder Julian Assange.

But it was not until a slightly later search, on December 29, 2009, that the accused definitely accessed "websites that had information pertaining to" the secret-spilling website, said Shaver.

The 800 or so web searches that Manning made included use of the keyword "Farah," relating to a US airstrike in Afghanistan said to have killed 100 civilians, a video of which was passed to WikiLeaks but never published. The video is expected to be discussed in evidence this week.

Manning, a homosexual, had also done searches relating to gender identity disorder, with which he has struggled, and the military's since renounced "Don't Ask, Don't Tell," policy on gays.

Manning's lead counsel David Coombs has so far sought to portray the accused as a naive but well-intentioned citizen who conducted the massive data dump because he wanted to start a public debate about US foreign policy.

However, the government contends that his actions aided US enemies. The trial is scheduled to last 12 weeks in total.