The FBI announced Tuesday that it is investigating an alleged leak of highly classified U.S. intelligence documents, days after assessments containing information about Israel’s potential plans for a retaliatory attack on Iran were published on an Iran-linked Telegram account.

“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 FBI said in a statement. “As this is an ongoing investigation, we have no further comment.”

White House National Security Council spokesman John Kirby told reporters Monday that it is unclear whether the documents were made public through a leak or a hack. He said the government does not expect additional classified information to be made public without authorization.

Early this month, Iran fired a barrage of nearly 200 ballistic missiles at Israel, a response to the killings of Hamas leader Ismail Haniyeh in Tehran in July and Hezbollah chief Hasan Nasrallah in Beirut in September. Tehran has been bracing for a retaliatory strike since then.

Kirby said Monday that President Joe Biden is “deeply concerned” about any leak of classified materials.

“That is not supposed to happen, and it’s unacceptable when it does,” Kirby said. “And you can rest assured that he will be actively monitoring the progress of the investigative effort to figure out how this happened, and obviously he’ll be very interested in hearing any mitigation measures and recommendations that come as a result of the investigative efforts and how to prevent it from happening again.”

Kirby said the Biden administration had been in communication with the Israeli government about the disclosure.

The intelligence assessment shows that the U.S. government believes Israel is preparing a major strike against Iran that would be a retaliation for a massive Iranian ballistic missile attack on Israel on Oct. 1. The Biden administration has urged Israeli Prime Minister Benjamin Netanyahu to carry out only a limited response that would not provoke yet another Iranian retaliation, in a bid to avoid a regional war.

U.S. officials have been pushing Netanyahu to focus on Iranian military targets, not Iran’s energy infrastructure or sites related to its nuclear program, which would far be more sensitive. Any attack on Iran’s oil facilities could also roil the U.S. election on Nov. 5 by creating a spike in energy prices.

The leaked assessment focused on U.S. observations of Israeli airfield activity on Tuesday and Wednesday of last week that it said were consistent with exercises to engage in long-range strikes that involve air-to-air refueling, something that would be necessary if Israeli fighter jets were to launch strikes on Iran. The assessment also discussed the movement of munitions carts on Israeli airfields. Two related documents were posted on the Telegram account, both dated Wednesday of last week.

An Israeli official said the government doesn’t consider the information revealed to have an impact on its planning for possible actions against Iran, but the episode did raise concern about security. “The content is not that significant,” said the official, who spoke on the condition of anonymity to discuss sensitive matters. “The fact that there was a leak is a lot more significant.”

The leak has unsettled the Biden administration, which remains jumpy from previous significant intelligence disclosures, including a slew of classified documents posted last year on the Discord messaging platform.

“We take these types of things very seriously, very, very seriously, and we investigate things if there is any type of incident,” Defense Secretary Lloyd Austin told reporters Monday in Poland. He declined to comment about any specific investigation.

Last year’s disclosure on Discord involved hundreds of classified documents. A member of the Massachusetts Air National Guard, Jack Teixeira, 22, pleaded guilty in March to a raft of related charges, including willful retention and transmission of national defense information that the government had classified as top secret.

The Air Force disciplined 15 other airmen in that case after a service investigation found that a “lack of supervision” and a “culture of complacency” in Teixeira’s unit at Otis Air National Guard Base on Cape Cod played a role in the leak. Teixeira, who held a junior rank of enlisted airman first class, had significant access to classified systems and had been observed acting suspiciously a few times without appropriate authorities being alerted, the investigation found.

One U.S. official said that although any intelligence leak is serious, the analysis that was published on the Iran-linked Telegram channel is likely to result in only limited diplomatic fallout, provided that no more material is released.

Analysis based on satellite imagery is inherently less sensitive than intercepted communications, information provided by spies burrowed inside foreign governments, or information shared by other countries with the U.S. intelligence community, the official said. Foreign governments know that the U.S. government has satellites that can do a good job of spotting just about anything that happens out in the open, the official said.

The leaked analysis also contained no actual images, just text, limiting the public’s ability to understand with precision the capabilities of U.S. spy satellites. The official spoke on the condition of anonymity to discuss internal considerations about the leak.

A spokeswoman for the Office of the Director of National Intelligence declined to comment.

Any investigation of a leak of this type will start with the originating agency — in this case the Defense Department’s National Geospatial-Intelligence Agency, which is charged with operating the intelligence community’s spy satellites — and then work outward in concentric circles of who would have had access to the classified document.

Even if the leaked assessment is based primarily on satellite imagery, it still offers a window into an unusually candid discussion of the U.S. intelligence community’s attitudes toward Israel, one former analyst said. And in saying that there are no indications that Israel intends to use a nuclear weapon against Iran, it contains a rare acknowledgment of Israel’s nuclear weapons program, which neither the United States nor the Israeli government has ever acknowledged publicly.

“The leaked assessment demonstrates how closely the intelligence community monitors Israeli military activity,” said Harrison Mann, a former analyst with the Defense Intelligence Agency who resigned to protest the Biden administration’s support for Israel’s war in Gaza.

It’s “a reminder that the administration does not actually trust Israel to share plans and major operations ahead of time. It demonstrates that the U.S. government acknowledges and monitors Israel’s nuclear forces.”

Mann also noted that the assessment shows that analysts “apparently used satellite imagery to track a ‘dolly,’ a cart used to transport missiles that’s about half as long and half as wide as a sedan, for long enough to infer that a specific type of missile was about to be loaded on Israeli fighters.”

John Hudson and Dan Lamothe in Washington; Missy Ryan in Rzeszow, Poland; and Steve Hendrix in Tel Aviv contributed to this report.

previous coverage

• Biden ‘deeply concerned’ about release of documents on Israel’s possible attack plans
• US investigates unauthorized release of classified documents on Israel’s attack plans