What we know about the Pentagon document leak

The FBI on Thursday arrested the person suspected of being behind the massive leak of highly classified Pentagon documents that revealed new details about the U.S.' allies and adversaries.

Why it matters: While officials are still assessing the validity of the documents, an interagency effort is being assembled to determine what impact the leak could have on U.S. national security.

• Below, Axios answers key questions about what is known about the leak, what the documents contained and how other countries have reacted to the revelations.

Where and when did the leak occur?

The leaked documents have appeared on various social media sites over the last week, including Twitter and Telegram, according to the New York Times, which first reported the leak.

• Some of the documents appeared on the platform Discord in early March while other documents, dating to January, appear to have been posted even earlier though it's not clear exactly when, according to Bellingcat.
• The documents were then spread to the anonymous message board 4chan and pro-Russian Telegram channels, before making their way to Twitter and major media outlets, per Bellingcat.

Who's behind the leak?

The latest: Attorney General Merrick Garland confirmed Thursday that the. the suspect is Jack Teixeira, a 21-year-old member of the Massachusetts Air National Guard.

• Teixeira was arrested in connection to “unauthorized removal, retention and transmission of classified national defense information,” Garland said, adding that the arrest occurred "without incident."

• The FBI has been conducting interviews in recent days after narrowing down a list of suspects as part of a fast-moving investigation, CNN reported.
• The FBI confirmed in a statement that the arrest took place at a residence in North Dighton, Massachusetts.
• The arrest exemplifies "our continued commitment to identifying, pursuing, and holding accountable those who betray our country's trust and put our national security at risk," the FBI said.
• Earlier Thursday, President Biden said the Justice Department was "getting close" to determining who was behind the leak.
• The person behind the leak was a "young, charismatic gun enthusiast" who is known as "OG" in a private chat room on the social media platform Discord, the Washington Post reported Wednesday.
• The leaker reportedly told the roughly two dozen chat group members that he worked on a military base where he spent part of his day inside a "secure facility that prohibited cellphones and other electronic devices."
• One member said that the leaker claimed he spent hours annotating classified documents to share with the group, the Post reported.
• The leaker was suspicious of law enforcement and the U.S. intelligence community and was prone to ranting about “government overreach," the Discord group member told the Post.
• They claimed that roughly half of the chat group members lived abroad and that those who appeared most interested in the classified material were primarily from the “Eastern Bloc and those post-Soviet countries."

Catch up quick: Hundreds, if not thousands, of government and military officials have the necessary security clearances to access the documents, the New York Times reported.

• The documents appeared online as photos of crumpled pieces of paper, laid on top of magazines and surrounded by household objects, suggesting they might have been hurriedly stuffed into a pocket and then photographed later, CNN reported.
• The tranche of leaked documents numbers roughly 100 pages, though at least one appeared to have been altered from the original at a later time, per the Times.
• The alterations overshot U.S. estimates for Ukrainian casualties in the war while underestimating Russian casualties, prompting Ukrainian officials and Russian bloggers to accuse the other side of an attempt at disinformation, per the Times.

What's in the documents?

The content of the documents varies, but includes U.S. assessments of the war in Ukraine dating to February and March, including territorial gains and ammunition shortages.

• The documents illustrate U.S. efforts to spy on Ukraine's government and military leaders and lays out the U.S.' extensive knowledge of Russian government intelligence — a fact that could hurt U.S. spy efforts if Russia figures out where the information is coming from, the Times noted.

• The documents also outline U.S. efforts to spy on its allies, Israel and South Korea.

Zoom in: One of the documents, a CIA update from March 1 sourced to signals intelligence, seemingly showed that leaders of Israel's intelligence agency encouraged Mossad officials and Israeli citizens to protest the government's proposed judicial reforms.

• Another report based on signals intelligence describes South Korea's concern that artillery shells requested by the U.S. to replenish its stockpiles could end up in Ukraine, the New York Times reported.
• One document slide suggested that a force of fewer than 100 special operations personnel from NATO members France, the U.S., the U.K. and Latvia was active in Ukraine, the Guardian reported.

How have other countries reacted?

Ukraine has questioned the validity of the documents, with Mykhailo Podolyak, an adviser to President Volodymyr Zelensky, claiming in a Telegram post that the leaks were an act of Russian disinformation.

• "The purpose of 'leaks' of allegedly secret data is obvious. To divert attention from the actual preparations for the next stage of the war.... To sow certain doubts and mutual suspicions between partners," Podolyak wrote.
• Kremlin spokesperson Dmitry Peskov told CNN that Russia doesn't have "the slightest doubt about direct or indirect involvement" by the U.S. and NATO in the war in Ukraine and that "this level of involvement is rising."

The big picture: Israeli Prime Minister Benjamin Netanyahu's office issued a statement rejecting the claims about Mossad, saying they were “mendacious and without any foundation whatsoever."

• A South Korean government official told reporters that the government was aware of reports about the leaks and that it planned to discuss the "issues raised" raised by the incident.

France has denied the allegations that its forces are on the ground in Ukraine.

• “There are no French forces engaged in operations in Ukraine,” the press team for Armed Forces Minister Sébastien Lecornu told Le Monde last weekend.
• "The documents cited do not come from the French military. We do not comment on documents whose source is uncertain," they added.

Editor's note: This story was updated with new details and developments.