CNN– The man arrested by the FBI in connection with a massive US classified documents leak was charged in Boston Friday with unauthorized retention and transmission of national defense information, as well as unauthorized removal of classified information and defense materials.

The 21-year-old suspect, Jack Teixeira, is a member of the Massachusetts Air National Guard and believed to be the leader of the group where a trove of classified documents had been posted online, officials told CNN.

Teixeira’s arrest Thursday marked the dramatic end to a fast-moving search by the US government to identify the person responsible for posting a trove of highly classified documents to a social media platform popular with video gamers.

The documents, some of which have been reviewed by CNN, included a wide range of highly classified information, including eavesdropping on key allies and adversaries and blunt assessments on the state of the Ukraine war.

The Biden administration is still grappling with the fallout from a major security breach that has rattled US officials, members of Congress and key allies in recent days.

President Joe Biden has been regularly briefed on the investigation, officials said, as well as the efforts by his top officials to engage with allies who have been identified within, or unsettled by, the content of the leaked information, according to one of the officials. Behind the scenes, it has been a reality that has loomed over a deeply personal and important foreign trip for Biden, one official acknowledged.

The Defense Department is also reviewing the matter and has taken steps to tighten the flow of such highly sensitive documents, limiting who across the government receives its highly classified daily intelligence briefs following the leak, officials said. Those briefs are normally available on any given day to hundreds, if not thousands, of people across the government.

The Pentagon has stood up an “interagency effort” to assess the impact of the leak, but US officials and close allies already fear the revelations could jeopardize sensitive sources and compromise important foreign relationships.

Congressional lawmakers have also expressed concerns about the apparent scope of the leak and sensitivity of the documents posted online, voicing frustration in recent days bout the lack of clarity they have received from US officials.

Here what we know about the leak so far:

What happened?

Teixeira began posting highly sensitive government information months ago on the social media platform Discord, according to a user who was interviewed by the FBI, but US officials only became aware of the leak in recent days.

According to a user of the Discord server who was interviewed by the FBI, Teixeira began posting information in December 2022, the affidavit released Friday says. He started posting photos of documents around January 2023.

But the initial public disclosure that the classified material had been posted online to a small Discord group came months later, prompting an urgent search for the person responsible.

It began with thousands of people who had access to the documents but investigators were able to quickly narrow the search to potential members of the chat group with evidence collected in the days immediately following the discovery of classified documents online by US officials.

Teixeira was under surveillance for at least a couple of days prior to his arrest by the FBI on Thursday, according to a US government source familiar with the case.

He made his first appearance in federal court in Boston Friday morning.

Prior to his arrest, Teixeira was concerned about getting caught transcribing documents at work, so he started taking them home, a member of his online chat group told the FBI, according to court documents.

Teixeira “began taking the documents to his residence and photographing them” due to his concern about getting caught, the group member told investigators, according to the court document.

The leaks included photos of crumpled documents lying on top of magazines and surrounded by other random objects, such as zip-close bags and Gorilla Glue, CNN has previously reported.

The classified documents – including some marked top secret, the highest level of classification – looked as if they had been hastily folded up and shoved into a pocket before being removed from a secure location, a source familiar with these kinds of documents told CNN.

Teixeira also allegedly searched a classified government database for the word “leak” on April 6, when reports began emerging publicly of classified information being posted online.

Who is Jack Teixeira?

Teixeira is a member of the Massachusetts Air National Guard, working as a low-ranking IT official in his role as a Cyber Transport Systems journeyman.

According to the Air Force, Cyber Transport Systems specialists are tasked with making sure the service’s “vast, global communications network” is “operating properly.”

Court documents show Teixeira held a Top Secret clearance since 2021.

He was assigned to 102nd Intelligence Wing is a “24/7 operational mission” that takes in intelligence from various sources and packages it into a product for some of the most senior military leaders around the globe, a defense official said.

Teixeira’s job was not to be the one packaging the intelligence for those senior commanders, but rather to work on the network on which that highly classified intelligence lived.

For that purpose, the official said Teixeira would be required to have a top secret/sensitive compartmented information (TS/SCI) clearance, in the instance that he was exposed to that level of intelligence.

He underwent “a very rigorous background check” to obtain that clearance, the official said.

Teixeira is also believed to be the head of obscure invite-only Discord chatroom called Thug Shaker Central, multiple US officials told CNN, where information from the classified documents was first posted months ago.

Teixeira grew up in the suburbs of Providence, Rhode Island, according to public records. He attended Dighton-Rehoboth High School in Massachusetts where he graduated in 2020, according to the superintendent of the regional school district.

CNN spoke to several of Teixeira’s former high school classmates who described him as “more of a loner” and said he a fascination with the military, guns and war.

Teixeira would sometimes wear camouflage to school, carried a “dictionary-sized book on guns” and behaved in a way that made some students uneasy, former classmates said.

“A lot of people were wary of him,” said Brooke Cleathero, who attended middle school and high school with Teixeira. “He was more of a loner, and having a fascination with war and guns made him off-putting to a lot of people.”

Teixeira didn’t behave in a manner that rose to the level where “people felt the need to report him,” another former classmate said, but “he made me nervous.”

The same student said she took his fascination with the military as a form of American nationalism and was therefore surprised by the allegations against him.

“I didn’t think he would be capable of doing something like this,” she said.

During a short detention hearing Friday, Teixeira was informed that he is facing two charges for allegedly posting classified documents online.

The two charges are unauthorized retention and transmission of national defense information and unauthorized removal and retention of classified documents or material, according to court documents.

The judge scheduled a detention hearing for Teixeira on Wednesday. He will remain detained until then. Teixeira did not enter a formal plea.

He also “maintained sensitive compartmented access (SCI) to other highly classified programs,” the affidavit released Friday says. Many of the leaked documents posted on the online server Discord were marked Top Secret.

What is in the documents?

CNN has reviewed 53 leaked documents, all of which appear to have been produced between mid-February and early March.

They contain a wide range of highly classified information – providing a rare window into how the US spies on allies and adversaries alike.

Some of the documents, which US officials say are authentic, expose the extent of US eavesdropping on key allies, including South Korea, Israel and Ukraine.

Others reveal the degree to which the US has penetrated the Russian Ministry of Defense and the Russian mercenary organization Wagner Group, largely through intercepted communications and human sources, which could now be cut off or put in danger.

Still others divulge key weaknesses in Ukrainian weaponry, air defense, and battalion sizes and readiness at a critical point in the war, as Ukrainian forces gear up to launch a counteroffensive against the Russians – and just as the US and Ukraine have begun to develop a more mutually trusting relationship over intelligence-sharing.

One document reveals that the US has been spying on Ukrainian President Volodymyr Zelensky. That is unsurprising, said a source close to Zelensky, but Ukrainian officials are deeply frustrated about the leak.

The US intelligence report, which is sourced to signals intelligence, says that Zelensky in late February “suggested striking Russian deployment locations in Russia’s Rostov Oblast” using unmanned aerial vehicles, since Ukraine does not have long-range weapons capable of reaching that far.

Signals intelligence includes intercepted communications and is broadly defined by the National Security Agency as “intelligence derived from electronic signals and systems used by foreign targets, such as communications systems, radars, and weapons systems.”

Yet another document describes, in remarkable detail, a conversation between two senior South Korean national security officials about concerns by the country’s National Security Council over a US request for ammunition.

The officials worried that supplying the ammunition, which the US would then send to Ukraine, would violate South Korea’s policy of not supplying lethal aid to countries at war. According to the document, one of the officials then suggested a way of getting around the policy without actually changing it – by selling the ammunition to Poland.

The document has already sparked controversy in Seoul, with South Korean officials telling reporters that they plan to raise the issue with Washington.

An intelligence report about Israel, meanwhile, has sparked outrage in Jerusalem. The report, produced by the CIA and sourced to signals intelligence, says that Israel’s main intelligence agency, the Mossad, had been encouraging protests against the country’s new government – “including several explicit calls to action,” the report alleges.

How are US allies reacting?

While US allies are aware that the US intelligence community collects information on friendly nations, diplomats from some of the countries mentioned told CNN it was frustrating – and harmful to the US reputation – to see that information exposed publicly.

US allies are doing damage assessments, scrambling to determine whether any of their own sources and methods have been compromised by the leak.

“We expect the US to share a damage assessment with us in the coming days, but we cannot wait for their assessment. Right now we are doing our own,” said an official from a country that is part of the Five Eyes intelligence-sharing arrangement with the US, which includes Australia, Canada, New Zealand and the United Kingdom.

“We are poring over these documents to figure out if any of the intelligence originated from our collection,” the official said.

A second Five Eyes nation official expressed concern about the leaked Ukraine war information handicapping the country on the battlefield.

The official also pointed out that it was alarming to see one of the documents from February titled “Russia-Ukraine: Battle for the Donbas Region Likely Heading for a Stalemate Throughout 2023.” The document notes the challenges with assessing the “endurance of Ukraine’s operations.”

“Gains for Ukraine will be hard to accomplish, but it does not help to have the private US assessment pointing to a likely yearlong stalemate revealed publicly,” the official said.

Mykhailo Podolyak, the adviser to the head of the Office of the President of Ukraine, said on his Telegram channel Friday that he believes the documents that have been disseminated are inauthentic, have “nothing to do with Ukraine’s real plans” and are based on “a large amount of fictitious information” disseminated by Russia.

Still, Ukraine has already altered some of its military plans because of the leak, a source close to Zelensky told CNN.

US government officials “are engaging with allies and partners at high levels over this including to reassure them of our commitment to safeguarding intelligence and the fidelity of securing our partnerships” following the leak, State Department principal deputy spokesperson Vedant Patel said Monday.

At the State Department, Deputy Secretary of State Wendy Sherman has been tapped to lead the diplomatic response, according to a US official familiar with the matter.

Patel would not go into details about which countries the US has engaged with, only saying “that work is ongoing.”

Key allies who are part of the Five Eyes intelligence-sharing arrangement (Australia, Canada, New Zealand and the United Kingdom) have not yet been given a briefing by the US on where they are on the damage assessment or efforts to identify the leaker, two Five Eyes diplomats told CNN on Monday.

One of the diplomats said they expect a briefing from US officials in the coming days and noted that the Easter holiday had slowed the pace of discussions in recent days.

Who is investigating?

The Justice Department has launched an investigation and the Defense Department is also reviewing the matter.

“The Department of Defense continues to review and assess the validity of the photographed documents that are circulating on social media sites and that appear to contain sensitive and highly classified material,” Pentagon deputy press secretary Sabrina Singh said in a statement over the weekend. “An interagency effort has been stood up, focused on assessing the impact these photographed documents could have on U.S. national security and on our Allies and partners.”

Singh added that US officials spoke with allies and partners over the weekend regarding the leak, and informed “relevant congressional committees.”

The Joint Staff, which comprises the Defense Department’s most senior uniformed leadership that advises the president, is examining its distribution lists to look at who gets these reports, a Defense official said. Many of the documents had markings indicating that they had been produced by the Joint Staff’s intelligence arm, known as J2, and appear to be briefing documents.

The Pentagon team working to determine the scale and scope of the leak includes the Defense Department’s legislative affairs, public affairs, policy, general counsel, intelligence and security, and joint staff offices, Assistant to the Secretary of Defense for Public Affairs Chris Meagher said Monday.

Asked if the government has any sense of who leaked the documents, National Security Council Coordinator for Strategic Communications John Kirby said Monday that the Department of Defense had referred the case to the Department of Justice for criminal investigation and directed questions to them.

“I’m not aware that they’ve come to any conclusions at this point about where they’re coming from,” Kirby said.

Asked if the administration believed the leak is contained or if there’s an ongoing threat, Kirby responded: “We don’t know. We truly don’t.”

This story was updated following the arrest and charging of Jack Teixeira over the leak.

CNN’s Alex Marquardt, Jeremy Herb, Jennifer Hansler and Haley Britzky, Blake Ellis, Scott Glover, Jeff Winter and Melanie Hicken contributed reporting.