Britain investigates "damaging" data linked to Snowden.

By Costas Pitas

LONDON, Aug 22 (Reuters) Britain has launched a criminal investigation after seizing potentially dangerous documents from the Brazilian partner of the journalist who revealed Edward Snowden's leaks about US and British electronic surveillance, a police legal representative said on Thursday.

The investigation is the latest twist in a surveillance scandal that has pitted US President Barack Obama against the Kremlin and led aides to British Prime Minister David Cameron to demand the return of secrets held by the British newspaper The Guardian.

Based on anti-terrorism laws, British police detained Brazilian David Miranda, partner of American journalist Glenn Greenwald, for nine hours at Heathrow Airport in London on Sunday.

Miranda, who was transporting documents between Greenwald and a journalist also linked to Snowden who lives in Berlin, was released without charges, but had his computer, phone, hard drive, and memory cards confiscated.

At a hearing in the High Court in London, where Miranda's lawyers' attempt to prevent British authorities from searching through the tens of thousands of documents contained on the devices was being assessed, a legal representative for the Metropolitan Police in London said that some of the information was dangerous.

"What has already been inspected contains, in the view of the police, highly sensitive material whose disclosure would be seriously detrimental to public safety and, therefore, the police have initiated a criminal investigation," said Jonathan Laidlaw.

Police declined to comment immediately on the subject of the criminal investigation. Miranda's lawyer, Gwendolen Morgan, told reporters that she knew very little about the investigation or what it was based on.

Greenwald, who lives in Rio de Janeiro and writes for the British newspaper The Guardian, has published articles based on documents leaked by Snowden, the former contractor for the US National Security Agency (NSA), who faces criminal charges in the United States.

FREEDOM OF THE PRESS

British security officials said that Snowden's leaks -- which revealed the scale of surveillance carried out by the US and Britain on all communications, from phone calls to emails and social media use -- have undermined national security and could put lives at risk.

But Miranda's detention and the British government's pressure on The Guardian newspaper have dragged Cameron into an international debate about press freedom and the powers of the security services.

Germany criticized Britain, while Russia, which granted Snowden temporary asylum, accused the British government of having dubious standards regarding press freedom.

The Brazilian government, which complained about Miranda's "unjustified" detention, asked Great Britain to return the electronic equipment seized from Miranda.

It was unclear what documents Miranda was carrying or what secrets might have led Britain to act in such a way. Greenwald said that England would regret its actions, which he said were an attempt to intimidate him.

Two judges of the British High Court, Jack Beatson and Kenneth Parker, ruled that British authorities could continue to examine Miranda's data for the sake of national security and investigate all possible links to terrorism.

Morgan, Miranda's lawyer, said the decision was a partial victory, but they can still appeal. The judges gave British authorities until August 30 to search the documents.

Miranda's lawyer also initiated legal action to have the judges determine that his detention was illegal.

(By Guy Faulconbridge and Michael Holden)