U.S. intelligence and defense officials took a risk in not locking down the classified information in war zones, said Thomas Ferguson, principal deputy under secretary for intelligence at the U.S. Department of Defense. The DOD allowed soldiers to transfer data between its own systems and allied networks so they could "rapidly" respond to conditions in Iraq, he said.

"The focus in the [Iraq] theater was speed and agility," he said. "We took that risk to allow not just Private Manning, but many people who are serving there, to move at that pace."

Manning, who worked in intelligence support in Iraq, allegedly gave hundreds of thousands of classified military and diplomatic documents to WikiLeaks during 2010. He was arrested last May and is awaiting trial on more than 20 charges.

Manning's access to DOD and U.S. Department of State documents came from a "complete breakdown of command authority," said Senator Scott Brown, a Massachusetts Republican.

Brown and other members of the Senate Homeland Security and Governmental Affairs Committee questioned how the DOD could allow a low-level private to allegedly burn hundreds of documents from the DOD's Secret Internet Protocol Router Network (SIPRNet) onto disks from his work computer without being detected. "That baffles me," said Senator Susan Collins, a Maine Republican.