U.S. will 'turn over every rock' to find source of leak, says Pentagon chief

U.S. Defense Secretary Lloyd Austin on Tuesday said the United States will investigate the recent purported leak of classified documents until the source of them was found.

Reuters has reviewed more than 50 of the documents, labeled "Secret" and "Top Secret," that first appeared on social media sites in March and supposedly reveal details of military capabilities of some U.S. allies and adversaries.

Reuters has not independently verified the documents' authenticity.

"We will continue to investigate and turn over every rock until we find the source of this and the extent of it," Austin said during a press conference at the State Department.

Austin, the first senior U.S. official to comment on the leak, said the Pentagon was aware that documents had been posted dated Feb. 28 and March 1, but was not sure if there were other documents that had been online before.

"These are things that we will find out as we continue to investigate," Austin added.

Investigators are working to determine what person or group might have had the ability and motivation to release the intelligence reports. The leaks could be the most damaging release of U.S. government information since the 2013 publication of thousands of documents on WikiLeaks.

Some of the most sensitive information is purportedly related to Ukraine's military capabilities and shortcomings and one document mentions the small number of Western special forces troops in the country.

The U.S. Department of Justice has opened a criminal investigation into the disclosure of the documents.

Some national security experts and U.S. officials say they suspect that the leaker could be American, given the breadth of topics covered by the documents. More theories could develop as the investigation progresses, they said.