"No one suspected a thing," Manning told a confidante afterward, according to a log of a computer chat published by Wired.com. "I didn't even have to hide anything."
"Weak servers, weak logging, weak physical security, weak counterintelligence, inattentive signal analysis," Manning wrote hacker Adrian Lamo explaining about how he was able stole the documents, according to Wired. "A perfect storm."
Wikileaks has pledged to release more than 250,000 documents, most, if not all of which, are believed to have been obtained by Manning. Already the site has published classified military records pertaining to the death toll in Iraq, and secret diplomatic cables about relations with foreign governments.
Keane said the large number of people accessing a huge quantity of data makes it nearly impossible for supervisors to monitor every key stroke.
"It's ambiguous at best, how the chain of command prevents this," he said.
"The Internet has decentralized the chain of command. An analyst doesn't need his supervisor's permission to access a computer. If you walked into [Afghan campaign commander Gen. David] Petraeus' operations center, you see hundreds of guys sitting at PCs. Nobody is sitting on top of them, looking over their shoulder," Keane said.
Beyond the logistics of monitoring, Keane said a supervisor should not be held responsible if a subordinate goes unexpectedly rogue. For instance, he said, a commander would not be held responsible if without warning a private threw a grenade into a tent full of soldiers.
However, he said, if Manning displayed warning signs and if the deception went on for an extended period of time, a supervisor might be held responsible.
"The military might decide not to fire supervisors and focus on the bigger question of what can we do to reduce the number of people with access to this information," said Torie Clarke, an ABC News consultant and former Department of Defense spokeswoman.
Former DOD officials said the military is still learning how to monitor its vast amounts of digital information, accessible to hundreds of thousands of service members and civilians.
The Pentagon still occasionally does physical spot checks, asking employees to open bags and briefcases to ensure encoded laptops and classified documents are not being taken out of the building, but that does little to staunch the flow of digital information.
Clarke echoed McCain, saying the number of people with access to intelligence has become dangerously large.
"Over the years we have cheapened the brand of intelligence. Too many people in the military, Congress and the administration have access. Part of that creates a casual environment in which people think they can get away with these things," she said.
The Pentagon has promised a review of its policies and access to its network and is already talking about banning flash drives, like the one Manning used.
"We should make sure that we have the fixes in, that we have confidence that that information will not be disseminated, except for those who need to know. And frankly, I'm worried about the additional information that Wikileaks apparently has," McCain said.
"Let's fix the problem, hold people responsible, and make sure this kind of thing never happens again," he said.