Probe into leaked U.S. intelligence documents expected to be painstaking

A probe into the leak of secret U.S. documents likely will require investigators to examine those who had access and scrutinize details like objects captured in photos of the materials, former U.S. officials told Reuters, as pressure grows to find the person or group responsible.

The Department of Justice opened a formal criminal probe last week after the matter was referred by the Pentagon, which is assessing the potential repercussions of what may be the most damaging release of classified U.S. information in years.

U.S. Defence Secretary Lloyd Austin on Tuesday said the United States will "turn over every rock" to find the source of the leak.

While leak investigations can take months or even years, investigators face intense pressure to find the source of this security breach given the possibility that more documents could be leaked.

But White House spokesperson John Kirby separately cautioned reporters that it was "difficult to know" how long it might be before answers are found.

"I think it would be foolish for anybody to guess how long that's going to take," Kirby said.

Reuters has reviewed more than 50 of the documents, labelled "Secret" and "Top Secret," but has not independently verified their authenticity.

Two former U.S. officials told Reuters that one of the investigators' first steps would be reviewing who had access to the dozens of documents, photographs of which were posted on social media platforms last month, or possibly earlier.

WATCH | Richard Fadden, former CSIS director, gives 4 reasons leak is problematic:

One potential clue: a number of documents are embossed with the Joint Chiefs of Staff emblem and one document is purportedly a daily intelligence update for the chairman of the Joint Chiefs of Staff and the secretary of defence. This could help investigators focus their effort, though many people could have had access to these documents.

Some images also depict printouts of documents with time stamps at the top right corners showing when they were printed.

That could be a key indicator because government classified computer systems keep logs of those who view and print documents, said Mark Zaid, a lawyer who practices national security law.

A former federal prosecutor, speaking on the condition of anonymity, said that once investigators narrow the list of people with access to the materials, they could seek "pen registers" that would show a history of outgoing phone calls made without revealing their content. They also could try obtaining search warrants for electronic cloud accounts and electronic devices, potentially giving access to private messages and documents.

Michael Atkinson, the U.S. intelligence community inspector general until 2020, said investigators may be able to discover the leaker's electronic fingerprints, given the large number of leaked documents along with the fact that they were shared on online forums.