The FBI is investigating the leak of classified documents that included top secret U.S. intelligence describing Israeli preparations for a possible attack on Iran, Fox News Digital confirmed. 

‘The FBI is investigating the alleged leak of classified documents and working closely with our partners in the Department of Defense and Intelligence Community,’ the bureau said in a statement. ‘As this is an ongoing investigation, we have no further comment.’ 

The Department of Defense has already confirmed it is investigating the unauthorized release.

White House National Security Council spokesman John Kirby told reporters on Monday that he did not have information on whether the unauthorized release was a result of a hack or an employee leak. 

‘We’re not exactly sure how these documents found their way into the public domain. I know the Department of Defense is investigating this,’ Kirby said. ‘I’m just not able to answer your question whether it was a leak or a hack. At this point, we’ll let the investigation pursue its logical course.’ 

‘We’re deeply concerned, and the president remains deeply concerned about any leakage of classified information into the public domain,’ Kirby said. ‘That is not supposed to happen. And it’s unacceptable when it does.’ 

Kirby said he did not have any indication that additional classified documents would find their way into the public domain and that the U.S. has been in communication with Israeli counterparts about the disclosure.

‘I’ll let the Israelis speak to if, what, how and when they decide to take additional military action in response to Iran’s Oct. 1 attack,’ Kirby said. ‘That’s really for them to speak to.’ 

The documents are descriptions of satellite images showing Israeli warplanes preparing for a strike and practicing air refueling, according to officials. There is no information in the leaked documents about what the targets are or what Israeli plans to strike.

‘These are NOT Israeli war plans for Iran,’ a senior U.S. defense official told Fox News.

The documents, which are marked top secret, were posted to the Telegram messaging app last week. 

The documents were attributed to the U.S. Geospatial Intelligence Agency and National Security Agency and noted that Israel was still moving military assets in place to conduct a military strike in response to Iran’s blistering ballistic missile attack on Oct. 1. The documents were sharable within the ‘Five Eyes,’ which are the U.S., Great Britain, Canada, New Zealand and Australia.

Fox News’ Jennifer Griffin and The Associated Press contributed to this report.