WASHINGTON (AP) — The FBI said Tuesday that it is investigating the unauthorized release of classified documents on Israel's preparation for a potential retaliatory attack on Iran.

White House national security spokesman John Kirby said Monday that the Biden administration is still not certain if the classified information was leaked or hacked but that officials don't have any indication at this point of "additional documents like this finding their way into the public domain."

The Associated Press reported Saturday that U.S. officials were investigating the release. The FBI confirmed the investigation for the first time on Tuesday and said in a statement that it is "working closely with our partners in the Department of Defense and Intelligence Community." It did not comment further.

The documents are attributed to the National Geospatial-Intelligence Agency and National Security Agency and note that Israel is still moving military assets in place to conduct a military strike in response to Iran's blistering ballistic missile attack on Oct. 1. They were shareable within the "Five Eyes," an intelligence alliance comprised of the U.S., Great Britain, Canada, New Zealand and Australia.

Marked top secret, the documents first appeared online Friday on the Telegram messaging app and quickly spread among Telegram channels popular with Iranians.