The Pentagon has referred the issue to the Department of Justice, which has opened a criminal investigation.
U.S. officials are scrambling to identify the source of the leaked classified documents that appeared online, with details ranging from Ukraine’s air defence to Israel's Mossad spy agency.
According to Micheal Mulroy, former senior Pentagon official in an interview with Reuters, the breadth of topics addressed in the documents, which touch on the war in Ukraine, China, the Middle East and Africa, suggest they may have been leaked by an American rather than an ally.
He said, "The focus now is on this being a U.S. leak, as many of the documents were only in U.S. hands.”
U.S. officials said the investigation is in its early stages and those running it have not ruled out the possibility that pro-Russian elements were behind the leak, which is seen as one of the most serious security breaches since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2013.
The Russian embassy in Washington and the Kremlin did not respond to requests for comment.
Following disclosure of the leak, more than 50 documents labeled "Secret" and "Top Secret" that first appeared last month on social media websites, beginning with Discord and 4Chan have been reviewed.
While some of the documents were posted weeks ago, their existence was first reported on Friday by the New York Times.
The authenticity of the leaked documents has not been verified yet. Some giving battlefield casualty estimates from Ukraine appeared to have been altered to minimize Russian losses. It is not clear why at least one is marked unclassified but includes top-secret information. Some documents are marked "NOFORN," meaning they cannot be released to foreign nationals, Reuters reports.
Two U.S. officials told Reuters on Sunday that they have not ruled out that the documents may have been doctored to mislead investigators as to their origin or to disseminate false information that may harm U.S. security interests.
The White House referred questions to the Pentagon.
In a statement on Sunday, the Pentagon said it was reviewing the validity of the photographed documents that "appear to contain sensitive and highly classified material."
The Pentagon has referred the issue to the Department of Justice, which has opened a criminal investigation.
One of the documents, dated February 23 and marked "Secret," outlines in detail how Ukraine's S-300 air defence systems would be depleted by May 2 at the current usage rate.
Such closely guarded information could be of use to Russian forces, and Ukraine said its president and top security officials met on Friday to discuss ways to prevent leaks.
Another document, marked "Top Secret" and from a CIA Intel update from March 1, says the Mossad intelligence agency was encouraging protests against Israeli Prime Minister Benjamin Netanyahu's plans to tighten controls on the Supreme Court.
The document said the U.S. learned this through signals intelligence, suggesting the United States had been spying on one of its most important allies in the Middle East.
In a statement on Sunday, Netanyahu's office described the assertion as "mendacious and without any foundation whatsoever."
Another document gave details of internal discussions among senior South Korean officials about U.S. pressure on Seoul to help supply weapons to Ukraine, and its policy of not doing so.
The office of South Korean President Yoon Suk Yeol said on Monday that fact checks on the documents are a priority and that it would request the U.S. to take "appropriate" steps after confirming details.
The official said investigators were looking at four or five theories, from a disgruntled employee to an insider threat who actively wanted to undermine U.S. national security interests.