The Plot Thickens.

The US intelligence analyst who leaked the footage that resulted in Wikileaks’ infamous “Collateral Murder” video has been outed by a hacker to whom he boasted of his actions online, and arrested by the Army’s Criminal Investigation Division. Wired has the story:

Manning was turned in late last month by a former computer hacker with whom he spoke online. In the course of their chats, Manning took credit for leaking a headline-making video of a helicopter attack that Wikileaks posted online in April. The video showed a deadly 2007 U.S. helicopter air strike in Baghdad that claimed the lives of several innocent civilians.

He said he also leaked three other items to Wikileaks: a separate video showing the notorious 2009 Garani air strike in Afghanistan that Wikileaks has previously acknowledged is in its possession; a classified Army document evaluating Wikileaks as a security threat, which the site posted in March; and a previously unreported breach consisting of 260,000 classified U.S. diplomatic cables that Manning described as exposing “almost criminal political back dealings.”

“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.

Manning’s arrest comes as Wikileaks has ratcheted up pressure against various governments over the years with embarrassing documents acquired through a global whistleblower network that is seemingly impervious to threats from adversaries. Its operations are hosted on servers in several countries, and it uses high-level encryption for its document submission process, providing secure anonymity for its sources and a safe haven from legal repercussions for itself. Since its launch in 2006, it has never outed a source through its own actions, either voluntarily or involuntarily.

Manning came to the attention of the FBI and Army investigators after he contacted former hacker Adrian Lamo late last month over instant messenger and e-mail… From the chat logs provided by Lamo, and examined by Wired.com, it appears Manning sensed a kindred spirit in the ex-hacker. He discussed personal issues that got him into trouble with his superiors and left him socially isolated, and said he had been demoted and was headed for an early discharge from the Army.

When Manning told Lamo that he leaked a quarter-million classified embassy cables, Lamo contacted the Army, and then met with Army CID investigators and the FBI at a Starbucks near his house in Carmichael, California, where he passed the agents a copy of the chat logs. At their second meeting with Lamo on May 27, FBI agents from the Oakland Field Office told the hacker that Manning had been arrested the day before in Iraq by Army CID investigators.

Lamo has contributed funds to Wikileaks in the past, and says he agonized over the decision to expose Manning — he says he’s frequently contacted by hackers who want to talk about their adventures, and he’s never considered reporting anyone before. The supposed diplomatic cable leak, however, made him believe Manning’s actions were genuinely dangerous to U.S. national security.

More thoughts on this as things develop. Interested in readers’ gut reactions.