BOSTON - A Massachusetts Air National Guardsman accused in the leak of highly classified military documents appeared in court Friday as prosecutors unsealed charges and revealed how billing records and interviews with social media comrades helped pinpoint the suspect.

Among the revelations: That the platform Discord provided information that helped lead the FBI to guardsman Jack Teixeira, and that Teixeira used his government computer to search for the word “leak” on the day last week when news media reports revealed that classified documents had been improperly disclosed.

President Joe Biden said the government was working to determine “the validity” of the leaked documents. In the meantime, he said in a White House statement, “I have directed our military and intelligence community to take steps to further secure and limit distribution of sensitive information, and our national security team is closely coordinating with our partners and allies.”

Friday’s new details about the highest-profile intelligence leak in years shed light on how investigators came to zero in on Teixeira, 21, even though a motive for the disclosures remains publicly unexplained. The Justice Department has said its investigation is continuing, and the Pentagon, which earlier in the week called it a serious national security breach, said it would conduct its own review of access to sensitive intelligence to prevent a similar leak in the future.

Teixeira appeared in federal court in Boston to face charges, under the Espionage Act, of unauthorized retention and transmission of classified national defense information. He did not enter a plea, but a federal magistrate judge ordered him jailed until a detention hearing next week.

The court appearance came less than 24 hours after Teixeira was arrested by heavily armed tactical agents on Thursday following a weeklong criminal investigation into the disclosure of the government records, a breach that exposed to the world unvarnished secret assessments on the war in Ukraine, the capabilities and geopolitical interests of other nations and other national security issues.

“This is not just about taking home documents. That is of course itself illegal. But this is about the transmission, both the unlawful retention and the transmission of the documents. Everyone knows here that the documents in the end were transmitted,” Attorney General Merrick Garland said Friday at the Justice Department.

Investigators believe Teixeira was the leader of an online private chat group on Discord, a social media platform popular with people playing online games and where Teixeira is believed to have posted for years about guns, games and his favorite memes.

The eight-page court affidavit details several steps in the FBI investigation, including an interview Monday with a Discord user familiar with Teixeira’s online posts. The document does not identify the person or say how he or she was located. But the source told the FBI that a username linked to Teixeira began posting what appeared to be classified information roughly in December in an online chat that the user said was meant for the discussion of geopolitical affairs and past and current wars.