LONDON (AFP) – Britain’s High Court ruled Thursday that material seized from the Brazilian partner of a journalist working to publish secrets from U.S. leaker Edward Snowden can only be partially examined by police.

Police in Britain launched a criminal investigation over the data, claiming the files it has seen are “highly sensitive” and would be “gravely injurious to public safety” if revealed.

David Miranda, 28, the partner and assistant of Guardian journalist Glenn Greenwald, was detained for nine hours at London Heathrow Airport under anti-terror laws as he changed planes on Sunday.

The Brazilian, who helped Greenwald work on the Snowden material, had his laptop, phone, memory cards and other electronic equipment confiscated by agents.

Snowden, a former U.S. National Security Agency contractor, leaked information on mass surveillance programs conducted by the NSA and Britain’s electronic eavesdropping agency GCHQ (Government Communications Headquarters).

He has been granted temporary asylum in Russia as he flees a U.S. bid to prosecute him.

Based on the material Snowden provided, British newspaper the Guardian has published a series of reports detailing the programmes, infuriating Washington.

Calling Thursday’s ruling a partial victory, Miranda’s lawyer Gwendolen Morgan said the Home Office, or interior ministry, and London police headquarters Scotland Yard now had seven days to prove there was a genuine threat to Britain’s security.

“The defendants are not to inspect, copy, disclose, transfer or distribute — whether domestically or to any foreign government or agency — or interfere with the materials obtained from Mr Miranda under Schedule 7 of the Terrorism Act, save for the purposes of protection of national security,” she said outside court.

“The very chilling effect of the implications of today’s judgement are something that journalists worldwide should be very concerned about,” she added.

Miranda will get his property back by midnight Saturday, Morgan said.

A lawyer for London’s Metropolitan Police force, Jonathan Laidlaw, told the High Court that a mass of data had been discovered by officers, who were still examining the material.

Tens of thousands of pages of material are in police hands, the court heard.

“That which has been inspected contains, in the view of the police, highly sensitive material disclosure of which would be gravely injurious to public safety,” he said. “Thus the police have now initiated a criminal investigation. I am not proposing to say anything else which might alert potential defendants here or abroad to the nature and the ambit of the criminal investigation which has now been started.”

Home Secretary Theresa May said before the hearing that the police were right to act if they thought Miranda was carrying material for Greenwald that could be useful to terrorists.

A Home Office spokesman said afterwards: “We are pleased the court has agreed that the police can examine the material as part of their criminal investigation insofar as it falls within the purposes of the original Schedule 7 examination and in order to protect national security.”

Separately, David Anderson, Britain’s independent reviewer of terror legislation, said Thursday he would launch an investigation to consider whether the anti-terror laws used to detain Miranda were “lawfully, appropriately and humanely used.”

On July 20, the left-liberal Guardian newspaper, under the supervision of GCHQ experts, destroyed the hard drives and memory chips on which its Snowden material had been saved.

Before Thursday’s hearing, Guardian editor Alan Rusbridger had informed government officials that copies of the encrypted files existed outside Britain and that the newspaper was not their sole recipient.

It has been confirmed that Cabinet Secretary Jeremy Heywood — Britain’s top civil servant and Prime Minister David Cameron’s most senior policy advisor — was sent to tell the Guardian they had to either destroy or return the material, or face legal action.

Cameron has faced calls to address parliament on the matter.

The destruction of the hard drive and Miranda’s detention have triggered unease in several countries over press freedom and human rights.