US Army intelligence analyst had reported that Manning had boasted online that he was going to reveal "the truth" about the war in Afghanistan.
Manning is currently awaiting court martial after his arrest in Baghdad in May 2010.
During online chats with a hacker, a man thought to be Manning said he had passed material relating to Afghanistan to Julian Assange, the founder of the Wikileaks website which leaked more than 91,000 secret documents to the media.
Manning is alleged to be a whistle-blower who used the online name Bradass87 when he contacted a high profile Californian computer hacker, Adrian Lamo, on May 21.
Over the following five days, Bradass87 held a series of online conversations with Lamo, in which he identified himself as "an army intelligence analyst, deployed to eastern Baghdad" with "unprecedented access to classified networks".
"I want people to see the truth. It's open diplomacy it's Climate gate with a global scope and breathtaking depth it's beautiful and horrifying; it's public data, it belongs in the public domain," said Bradass87.
Lamo had said he had no doubt Manning was behind the vast amount of leaked material from Afghanistan, though he strongly suspected the young analyst from Maryland could not have acted alone.