WASHINGTON — Pentagon leaders Thursday promised a thorough investigation into the release of 91,000 pages of sensitive military documents to the website, saying the security breach will force changes in operational tactics and intelligence gathering in Afghanistan.

Joint Chiefs of Staff Chairman Adm. Mike Mullen angrily attacked the breach as a reckless move, by both the unknown Defense Department employee and WikiLeaks founder Julian Assange.

“Mr. Assange can say whatever he likes about the greater good of what he and his source are doing,” Mullen said. "But the truth is they might already have on their hands the blood of some young soldier or that of an Afghan family."

Defense Secretary Robert Gates has asked FBI and Justice Department officials to help with the investigation into the release. He also said the incident would likely force changes in the way sensitive documents are handled in the combat zone, an area where he admitted restrictions have been lax in the past.

Gates did not rule out criminal action against Assange, but said defense officials have not made any such moves so far. Earlier this month, the military arrested Army Pfc. Bradley Manning, an intelligence specialist, and charged him with passing classified information to outside groups while he was stationed in Iraq.

He said defense officials are confident that the leak did not occur at a rear headquarters or in the U.S. — if so, “we would’ve detected it,” he said. As a result, Gen. David Petraeus and Gen. Lloyd Austin, the heads of U.S. operations in Afghanistan and Iraq, respectively, will have to reassess how sensitive information is transported and shared.

The challenge now will be “to strike the right balance between security and providing frontline troops with the information they need,” Gates said.

Both men said that despite critics claims that the reports show that the U.S. strategy in the region is failing, the breach will not result in any large-scale changes to the military’s approach in the region.

Gates said most of the information contained in the reports is either mundane or widely known, but the details of how and where it was collected could create battlefield consequences that are “potentially severe and dangerous.”

The release may have exposed the names of Afghan informants or potential allies within local villages, and could discourage other Afghans from stepping forward to help out U.S. troops.

“Will people whose lives are on the line trust us to keep their identities secret?” he said. “It seems to me, as a result of this massive breach of security, we have considerable repair work to do.”

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