Dozens of pages of classified documents circulating online are being described as the most serious intelligence leak in years. US officials say they are investigating how the materials were posted on a gaming platform and the impact they’ve had.

The FBI is zeroing in on a government office where it believes the leaked intelligence documents on Israel’s preparations for a possible strike on Iran were printed, two US sources familiar with the matter told CNN. Several US departments have tightened their tracking of when employees print sensitive documents, in light of the incident. The Pentagon is also reviewing how it shares information with allies and is restricting who receives daily intelligence briefings following the leak, officials said.

While it’s quietly accepted that the US spies on key allies, diplomats from some of the countries mentioned in the leaked documents told CNN it was frustrating and damaging to their reputation to see US intelligence reports about them made public. They fear their own sensitive sources and operations may have been compromised by the disclosures.

The FBI has identified one of the suspects allegedly behind the leaks. Airman first class Jack Teixeira of the Massachusetts Air National Guard is believed to have photographed the printouts of the leaked documents at his parents’ home, and then shared them on instant messaging app Telegram and 4chan, according to an affidavit released by the FBI on Friday. He has been charged with unauthorized dissemination of classified information. The revelations have rattled some congressional lawmakers, and they’ve urged FBI Director Chris Wray to prosecute the suspect to the fullest extent of the law.