Matthew Chance describes what's inside leaked documents

(CNN) — The US is investigating a leak of highly classified US intelligence about Israel’s plans for retaliation against Iran, according to three people familiar with the matter. One of the people familiar confirmed the documents’ authenticity.

The leak is “deeply concerning,” a US official told CNN.

The documents, dated October 15 and 16, began circulating online Friday after being posted on Telegram by an account called “Middle East Spectator.”

They are marked top secret and have markings indicating they are meant to be seen only by the US and its “Five Eyes” allies — Australia, Canada, New Zealand and the United Kingdom.

They describe preparations Israel appears to be making for a strike against Iran. One of the documents, which says it was compiled by the National Geospatial-Intelligence Agency, says the plans involve Israel moving munitions around.

Another document says it is sourced to the National Security Agency and outlines Israeli air force exercises involving air-to-surface missiles, also believed to be in preparation for a strike on Iran. CNN is not quoting directly from or showing the documents.

A US official said the investigation is examining who had access to the alleged Pentagon document. Any such leak would automatically trigger an investigation by the FBI alongside the Pentagon and US intelligence agencies. The FBI declined to comment.

The leak comes at an extremely sensitive moment in US-Israeli relations and is bound to anger the Israelis, who have been preparing to strike Iran in response to Iran’s missile barrage on October 1. One of the documents also suggests something that Israel has always declined to confirm publicly: that the country has nuclear weapons. The document says the US has not seen any indications that Israel plans to use a nuclear weapon against Iran.

“If it is true that Israeli tactical plans to respond to Iran’s attack on October 1 have been leaked, it is a serious breach,” said Mick Mulroy, former deputy assistant secretary of defense for the Middle East and a retired CIA officer.

Mulroy added that “the future coordination between the US and Israel could be challenged as well. Trust is a key component in the relationship, and depending on how this was leaked that trust could be eroded.”

The National Security Council referred CNN to the Office of the Director of National Intelligence and the Pentagon for comment. The Pentagon and National Geospatial-Intelligence Agency declined to comment. CNN has reached out to the National Security Agency for comment.

Another US official said that “these two documents are bad, but not horrible. The concern is if there are more.”

It is not clear how the documents became public, nor whether they were hacked or deliberately leaked. The US is already on high alert about Iranian hacking campaigns — US intelligence agencies said in August that Iran had hacked documents belonging to Donald Trump’s campaign.

Axios first reported on the leaked documents Saturday.

A major leak of US intelligence last year also strained the US’ relationships with allies and partners, including South Korea and Ukraine, after a 21-year-old Air National Guardsman posted highly classified information on the social media platform Discord.

This story has been updated with additional information.

CNN’s Evan Perez and Katie Bo Lillis contributed to this report.

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