Bradley Manning, the soldier accused of the biggest intelligence leak in US history, came face-to-face on Tuesday with the man who turned him in to military authorities.

Adrian Lamo, a former computer hacker, was giving evidence at the court martial of Manning, whom he had never met but whose life he changed dramatically by informing on him to counter-intelligence officers.

Manning’s defence lawyer used the cross-examination of Lamo to explore the soldier’s motivations and state of mind at the time he transmitted a huge amount of secret information to the open information website, Wikileaks.

The court heard that Manning had indicated to Lamo that he had decided to leak the information as a way of instigating a worldwide debate and disseminating the truth.

Lamo said that he had engaged over the space of six days from about 20 May 2010 in a web chat with Manning. The soldier had originally contacted him via a Gmail and military email address, but at Lamo’s suggestion they switched to an encrypted AOL instant messaging channel to keep their conversation private.

Lamo, dressed in black and with a silver cross around his neck, told the court that he had cross-referenced Manning’s email, AOL chat and Facebook page to ascertain his identity and association with the military.

David Coombs, Manning’s principal civilian lawyer, used the cross-examination to elucidate the soldier’s state of mind at the time of the leak. The questioning was subject to tetchy exchanges with prosecution lawyers who objected at several points that it was drifting into hearsay.

Lamo told Coombs that Manning had indicated in the course of the conversations that he knew the witness was a “grey-hat hacker” – ie he broke into computer systems to test their security weaknesses; that he was involved in the LBGT community; and that he had donated to WikiLeaks. “He told you he reached out to you as someone who might understand him?” Coombs asked.

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