Bradly Manning was arrested after boasting in instant messages and emails to a high-profile former hacker, Adrian Lamo, that he passed on the video, which shows a US air strike that killed a dozen people, including two Iraqis working for Reuters news agency. The video, named Collateral Murder by WikiLeaks, proved to be highly embarrassing to the US military after the air crew was heard falsely claiming to have encountered a firefight in Baghdad and then laughing at the dead .
Lamo told Wired, which broke the story of the arrest, that he went to the military and FBI about Manning "because lives were in danger" after the soldier also boasted of leaking thousands of pages of diplomatic cables.
Manning, 22, who had top-secret clearance, was arrested at a base east of Baghdad a fortnight ago. He is being held in Kuwait.
Manning contacted Lamo last month after reading a Wired article on the former hacker, who was convicted in 2004 of breaking in to computers at the New York Times, Yahoo and Microsoft.
"If you had unprecedented access to classified networks 14 hours a day, seven days a week, for eight-plus months, what would you do?" Manning asked.
Manning wrote to Lamo that he contacted the director of WikiLeaks, Julian Assange, in November after the site posted 500,000 pager messages sent immediately after the 9/11 attacks.
"I immediately recognised that they were from an NSA [National Security Agency] database, and I felt comfortable enough to come forward," he said.
The soldier said the video of the helicopters shooting in Baghdad had worried him.
"At first glance, it was just a bunch of guys getting shot up by a helicopter," Manning wrote of the video. "No big deal – about two dozen more where that came from, right? But something struck me as odd with the van thing, and also the fact it was being stored in a JAG [Judge Advocate General] officer's directory. So I looked into it."
Manning gave the video to Wikileaks in February, which made it public two months later after breaking the encryption.
But Lamo said what disturbed him was a boast by Manning that he had sent 260,000 pages of confidential diplomatic cables to WikiLeaks.
"Hillary Clinton and several thousand diplomats around the world are going to have a heart attack when they wake up one morning and find an entire repository of classified foreign policy is available, in searchable format, to the public," Manning wrote.
Lamo said he had met military investigators and the FBI because he feared the leak of diplomatic cables endangered US security.
"I wouldn't have done this if lives weren't in danger," Lamo told Wired. "He was in a war zone and basically trying to vacuum up as much classified information as he could, and [was] just throwing it up into the air."
Manning claimed to have leaked a second video of an air strike a year ago near Garani village, in Afghanistan, which the local authorities said killed nearly 100 people, most of them children. The Pentagon at first said it would release the video but then backed off.
WikiLeaks responded on Twitter by calling Lamo a notorious felon, informer and manipulator, and warning against believing his account. It also said it has no record of the 260,000 documents Manning said he had leaked.
Source: The Guardian