US Defense Secretary Lloyd Austin on Tuesday said the United States will investigate the recent purported leak of classified documents until the source is found.

“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.

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 US government information since the 2013 publication of thousands of documents on WikiLeaks.

Milancy Harris, deputy undersecretary of defense for intelligence and security, is leading the Pentagon’s review to assess the potential impact of the leaked documents, a U.S. official told Reuters.

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