LONDON – Britain has launched a criminal investigation into Edward Snowden’s leak of classified material to the Guardian newspaper and is sifting through documents it seized from the partner of one of the paper’s journalists, a government lawyer said Thursday.

The revelation by lawyer Jonathan Laidlaw came at London’s High Court, where attorneys for David Miranda — the partner of Guardian journalist Glenn Greenwald — unsuccessfully sued to stop police from combing through digital material seized from him Sunday at Heathrow Airport.

It was the British government’s first mention of a criminal investigation linked to the seized material, which included a laptop, cellphone, DVDs and memory sticks.

Greenwald has been at the center of the Guardian’s disclosures about the National Security Agency, which have pulled back the curtain on the American government’s secret domestic espionage program. Miranda, a 28-year-old Brazilian student, was detained for nearly nine hours as he flew through the London airport after meeting in Germany with a journalist working with Greenwald.

Laidlaw said British police had already begun scanning through Miranda’s tens of thousands of pages of documents, which he described as “highly sensitive.”

“The disclosure of (the material) would be gravely injurious to public safety and thus the police have now initiated a criminal investigation,” Laidlaw said.

The Guardian said it was unaware of any criminal investigation and had yet to hear from police about the matter. London’s Metropolitan Police said in a statement that its counterterrorism command had only just launched the inquiry Thursday.

It was not immediately clear what link a British investigation would have to American efforts to prosecute Snowden, who has won temporary asylum in Russia and is still being sought by the U.S. government on charges of espionage.

Laidlaw appeared to keep his comments deliberately vague, telling the court: “I am not proposing to say anything else which may alert potential defendants here or abroad to the nature and the ambit of the criminal investigation which has now begun.”

The High Court on Thursday largely ruled in favor of the government, saying officials could continue to sift through the seized data on national security grounds until a court hearing Aug. 30 examines the government’s reasoning.