LONDON (AP) -- 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 lawyers 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. "There is an absolutely compelling reason to permit this investigation to continue."
The exact target of the investigation was not clear.
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 Barack Obama'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.
Judges Jack Beatson and Kenneth Parker did issue a partial injunction blocking the government from examining or sharing the data for any other purpose, something that would prevent police, for example, from searching Miranda's hard drive for evidence of more mundane wrongdoing.
An independent legal expert said that at first blush the injunction fell far short of Miranda's goals.
"The object they were seeking as quickly as possible was the prevention of the police going through this material and they haven't achieved that," said Cathryn McGahey, a London lawyer who has handled civil liberties cases.
She noted that the government often has the upper hand in preliminary hearings if it can argue that national security is at stake.
"Once you've got a reputable state entity saying that, it's extremely hard for the court to go behind it," McGahey said.
Even if a future court hearing finds that Miranda was detained improperly or that his documents must be returned, the government would still have had the chance to peek at what's inside — something his lawyers had described as a blow to journalistic integrity.
"Confidentiality, once lost, can clearly never be restored," attorney Gwendolen Morgan said in a statement before the hearing.
Greenwald had said earlier this week that Miranda's documents were "protected by very advanced and heavy forms of encryption" — software that scrambles data so it's unreadable by anyone without a digital key.
Laidlaw, the government attorney, didn't explicitly contradict that, but his comment Thursday that police were already "partway through" the material suggested that British authorities had managed to read the files regardless.
The Guardian declined to comment on the encryption issue.