WikiLeaks founder Julian Assange will not willingly travel to the United States to face charges filed under seal against him, one of his lawyers said, foreshadowing a possible fight over extradition for a central figure in the U.S. special counsel’s Russia-Trump investigation.

Assange, who has taken cover in the Ecuadorian Embassy in London, where he has been granted asylum, has speculated publicly for years that the Justice Department had brought secret criminal charges against him for revealing highly sensitive government information on his website.

That hypothesis appeared closer to reality after prosecutors, in an errant court filing in an unrelated case, inadvertently revealed the existence of sealed charges. The filing, discovered Thursday night, said the charges and arrest warrant “would need to remain sealed until Assange is arrested in connection with the charges in the criminal complaint and can therefore no longer evade or avoid arrest and extradition in this matter.”

A person familiar with the matter, speaking on condition of anonymity because the case had not been made public, confirmed that charges had been filed under seal. The exact charges Assange faces and when they might be unsealed remained uncertain Friday.

Any charges against him could help illuminate whether Russia coordinated with the Trump campaign to sway the 2016 presidential election. They also would suggest that, after years of internal Justice Department wrangling, prosecutors have decided to take a more aggressive tack against WikiLeaks.

A criminal case also holds the potential to expose the practices of a radical transparency activist who has been under U.S. government scrutiny for years and at the center of some of the most explosive disclosures of stolen information in the last decade.

Those include thousands of military and State Department cables from Army Pvt. Bradley (now Chelsea) Manning, secret CIA hacking tools, and most recently and notoriously, Democratic emails that were published in the weeks before the 2016 presidential election and that U.S. intelligence officials say had been hacked by Russia.

Federal special counsel Robert Mueller, who has already charged 12 Russian military intelligence officers with hacking, has been investigating whether any Trump associates had advance knowledge of the stolen emails.