Defense attorneys grilled US Army investigators Saturday about the evidence against Bradley Manning and the sexual identity of the intelligence analyst accused of spilling US secrets to WikiLeaks.

In cross-examination, defense attorneys for Manning, who was spending his 24th birthday in court, questioned the witnesses about whether they had come across any evidence that the US Army private had "gender identity disorder."

The line of questioning appeared to be an apparent bid by Manning's defense team to establish that he may have been suffering from mental health issues at the time of his arrest over 18 months ago.

Manning is suspected of downloading 260,000 US diplomatic cables, videos of US air strikes and US military reports from Afghanistan and Iraq while serving as a low-ranking intelligence analyst in Iraq, and then providing them to anti-secrecy group WikiLeaks.

According to one of Manning's two military-appointed lawyers, the video of a deadly US Apache helicopter air strike on a group of men in Iraq that Manning is suspected of supplying to WikiLeaks was in fact unclassified, but an Army investigator said the video was believed to have been classified at the time.

The men, who the Apache crew had thought were carrying a rocket launcher, in fact included a Reuters staff member with a television camera.

Attorney Major Matthew Kemkes asked Special Agent Toni Graham of the Army's Criminal Investigation Command whether she had come across "any information that discussed gender identity disorder" during a search of Manning's possessions.

"I do remember several things on homosexuality and gender disorder," said Graham, who participated in an investigation leading to Manning's initial arrest. But she added that she was "not focused on that particular aspect of his life."

When prosecutors objected to the line of questioning, Kemkes said it was relevant because it goes to Manning's "state of mind."

"If the accused is facing gender identity disorder and is maintaining medical articles and pamphlets in his (housing unit), then that's relevant to his state of mind," he said.

And Graham acknowledged that Manning apparently had a "very limited" number of friends.

Another lawyer for Manning, Captain Paul Bouchard, sought to establish that other soldiers could have had access to the machines apparently used to send the files to Wikileaks.

"You don't know that those computers and other devices were used by other soldiers?" Bouchard asked Special Agent Calder Robertson of the Army's Computer Crime Investigative Unit, who said that he could not be entirely certain.

Robertson, who examined Manning's personal laptop and other devices following his arrest, was asked by Bouchard whether he had "come across any evidence that PFC Manning suffers from gender identity disorder," was "trying to create an alter ego called Breanna Manning" or was "emotionally troubled."

Robertson noted that Manning had referred to himself as "troubled" in instant message chats with Adrian Lamo, the former computer hacker who turned the intelligence analyst over to the US authorities.

Manning is facing charges that could potentially send him to prison for the rest of his life and the hearing on this sprawling US military base is being held to decide whether he should face a court-martial.

The pre-trial hearing, which began Friday and could last up to a week, is being held in an austere courthouse at Fort Meade, headquarters of the top secret National Security Agency.

Manning's supporters have been holding vigils and rallies outside the gates of Fort Meade and a number of his backers are attending the hearing.

Outside the court on Saturday some 200 activists protested the trial, denouncing US authorities for suppressing information Manning allegedly sought to expose.

The ex-computer hacker is being persecuted "for getting the truth out, especially when the truth involves the US violating human rights," said protester Chris Hager, wearing an orange jumpsuit and holding a sign with the message "I am Bradley Manning."

"They want to make such an example of him, so it would be a major warning for others do go in his footsteps," Hager said of the trial and Manning's treatment since his arrest.