The Defense Department and FBI are investigating suspicions of a major intelligence leak of documents showing a U.S. analysis of Israeli military preparations

WASHINGTON— The FBI and Defense Department are investigating a potentially significant intelligence breach and disclosing classified information about Israel's plans to attack Iran, U.S. officials said Monday.

Here's what we know so far:

Two leaked documents reveal a US analysis of Israel's plans against Iran

Earlier this month, Israel announced that it would take retaliatory measures against Iran for the October 1 rocket attack, in which Iran fired 200 rockets at various targets inside Israel. However, it was not clear how or when Israel would respond.

Amid speculation about Israel's next move, two documents marked “top secret” surfaced on social media last week, purporting to show a U.S. military analysis of Israeli operations.

One document purportedly comes from the National Geospatial-Intelligence Agency (NGA), a military agency that collects, analyzes and distributes information from satellite and aerial images. The second claims to contain information from the National Security Agency. (NSA).

ABC News does not directly quote or show the documents because they appear to detail the movement of Israeli military equipment and ammunition that could be used in a possible attack.

One of the documents claimed that Israel could attack Iran without the US seeing further visual cues from above.

The FBI was investigating the leak as part of a criminal investigation, according to a person familiar with the investigation. The White House said Monday that the Defense Department was also investigating the disclosure and that officials had discussed the alleged breach with Israel.

“We are deeply concerned, and the President remains deeply concerned, about the release of classified information to the public. “This cannot be allowed to happen, and it is unacceptable if it does happen, so he is deeply concerned about it,” White House national security spokesman John Kirby told reporters on Monday.

Kirby said there was no indication that more documents could be released, but “we will definitely keep our antenna up and our eyes peeled for possible future revelations.”

When asked about the leak on Monday, Defense Secretary Lloyd Austin declined to provide details other than to say: “We take things like this very seriously. Very, very serious.”

It is not yet clear whether the documents were leaked by an insider or stolen by a hacker

Both documents bear markings indicating that, if authentic, they would have been passed to the so-called Five Eyes, the information-sharing alliance consisting of the United States, Australia, Canada, New Zealand and the United Kingdom.

The documents appeared on a Telegram channel called Middle East Spectator, an anonymous blog that frequently publishes pro-Iranian content. The station's administrator told ABC News that he obtained the documents through an acquaintance who received them from an unknown source. The administrator denied being affiliated with any government.

It is possible that a foreign entity such as the Iranian government stole the documents by hacking the systems of countries that have access to the intelligence information. But at least one official familiar with the investigation said the probe would aggressively focus on anyone working for or with the U.S. government who had access to the material.

When asked about the possibility that the leak came from an insider, Kirby declined to speculate.

“We’re letting the investigation take its logical course there,” Kirby said.

Earlier this year, Massachusetts Air National Guardsman Jack Teixeira pleaded guilty to six counts of intentionally storing and disclosing national defense information. According to prosecutors, Teixeira leaked information in a Discord chat room about the type of equipment the U.S. was sending to Ukraine, troop movements in Ukraine and a plan by a foreign adversary to attack U.S. forces abroad.

What's next?

Lawmakers will likely have serious questions about how another major public disclosure of classified information could occur again after the Teixeira case.

House Speaker Mike Johnson told CNN on Sunday that lawmakers would receive a confidential briefing.

Another question is whether the leak forced Israel to adjust its military plans.

“If it is true that Israel's tactical plans for responding to Iran's attack on October 1 were leaked, that is a serious violation,” said Mick Mulroy, a national security and defense contributor at ABC News who served as deputy deputy Minister of Defense acted for the Middle East.

“Anyone who has access to this information has an obligation to keep it secure,” Mulroy said. “The men and women of the IDF (Israel Defense Forces) This could jeopardize the organization that would carry out this mission, and also jeopardize future US-Israel coordination.”

ABC News' Christopher Looft and Luke Barr contributed to this report.