Highly secret intel personal data on Ukraine’s air defenses and Israel’s Mossad intelligence agency have emerged online, prompting U.S. officials to search for the leaker, which some analysts believe is an American.

The records include Ukraine, China, the Middle East, and Africa, suggesting an American leaker.

“The focus now is on this being a U.S. leak,” former senior Military officer Michael Mulroy told Reuters.

The leak, one of the worst security breaches since WikiLeaks released 700,000 documents, videos, and diplomatic cables in 2013, is under investigation by U.S. officials, who have not ruled out pro-Russian involvement.

The Kremlin and the Russian embassy in Washington declined to comment.

After the breach, Reuters studied more than 50 “Secret” and “Top Secret” papers that originally emerged on Discord and 4Chan last month—the New York Times revealed on Friday that some papers were uploaded weeks earlier.

Reuters has not validated the papers. Certain Ukrainian battlefield mortality counts were manipulated to downplay Russian casualties. Why one is unclassified yet contains top-secret information is unclear. Foreigners cannot receive “NOFORN” papers.

On Sunday, two U.S. officials told Reuters that the documents might have been doctored to mislead investigators or spread false information that may compromise U.S. security interests.

The White House deferred to the Pentagon.

The Pentagon said Sunday it was investigating the photographed documents that “appear to contain sensitive and highly secret material.”

The Department of Justice started a criminal inquiry after the Pentagon submitted the matter.

The “Hidden” February 23 memo details how Ukraine’s S-300 air defense systems would be exhausted by May 2 at the present usage rate.

Ukraine’s president and senior security officials gathered Friday to explore measures to avoid leaks of such sensitive material, which Russian troops may utilize.

Another “Top Secret” CIA Intel bulletin dated March 1 states that the Mossad intelligence organization was promoting rallies against Israeli Prime Minister Benjamin Netanyahu’s Supreme Court reforms.

The memo indicated the U.S. learned this through signals intelligence, implying it was snooping on one of its most significant Middle Eastern friends.

Netanyahu’s administration called the claim “mendacious and without any substance whatsoever” Sunday.

In another paper, senior South Korean officials discussed U.S. pressure on Seoul to deliver weapons to Ukraine and its refusal.

On Monday, South Korean President Yoon Suk Yeol’s office said it would propose “appropriate” action from the U.S. after fact-checking the papers.

Yoon’s administration warned that “disrupting the alliance will suffer sanctions” if the papers were faked or tampered with.

The espionage claims were called a breach of national sovereignty and a grave security disaster by some Democratic Party MPs.

In a united statement, legislators demanded a thorough inquiry and urged no repeats.

The Pentagon hasn’t commented on particular papers, including allies’ monitoring.

Two anonymous U.S. officials said the Pentagon and intelligence agencies were concerned about the leak, although the documents represented a snapshot from more than a month ago.

Two individuals said the military and intelligence services were reviewing their internal intelligence-sharing methods.

One insider told Reuters that investigators are investigating why a U.S. official or group of officials leaked such sensitive material.

The individual said investigators considered four or five ideas, from a disgruntled employee to an insider threat who sought to harm U.S. national security.