LONDON - Glenn Greenwald, the journalist who first published the contents of classified documents provided by ex-NSA contractor Edward Snowden, said Monday that Britain will regret detaining and questioning his partner.
"I will be more aggressive in my reporting from now,” he told reporters in Portuguese at Rio de Janeiro’s airport, where he met his boyfriend, David Miranda, who had flown from London to Brazil.
Greenwald told reporters he has many more documents to report on, including ones about the UK. He said he thinks British authorities would come to regret their actions.
Brazilian national Miranda, 28, who lives with Greenwald in Rio, was held at London’s Heathrow Airport for nine hours Sunday by authorities using powers granted under local anti-terrorism laws.
Greenwald, who writes for the U.K.-based Guardian newspaper, said officials questioned Miranda about the stories published as a result of Snowden's leaked documents.
"They wanted to intimidate our journalism, to show that they have power and will not remain passive but will attack us more intensely if we continue publishing their secrets," he said.
Greenwald also accused the British agents of asking Miranda for the password to his laptop computer, and what he knew about the documents that Snowden had provided to Greenwald and documentary filmmaker Laura Poitras.
Miranda was transiting through London on his way back from Berlin, where he had met with Poitras.
The Guardian said the agents also confiscated Miranda's laptop, mobile phone, camera, memory sticks, DVDs and games consoles.
"We were dismayed that the partner of a Guardian journalist who has been writing about the security services was detained for nearly nine hours while passing through Heathrow airport," the Guardian said in a statement provided to NBC News.
"We are urgently seeking clarification from the British authorities."
Brazil's government complained about Miranda's detention in a statement on Sunday that said the use of the British anti-terrorism law was unjustified.
"This measure is without justification since it involves an individual against whom there are no charges that can legitimate the use of that legislation," they said the statement. "The Brazilian government expects that incidents such as the one that happened to the Brazilian citizen today do not repeat."
There was also disquiet in Britain about the move. Tom Watson, member of parliament for the opposition Labour Party, called it "an embarrassment to the Government."
"What I think we are going to see is this is sort of the intelligence services overstepping the mark - they are clearly trying to intimidate Glenn Greenwald - and that's an attack on journalism,” Watson told the BBC.
"I think politics needs to intervene to make sure it doesn't happen again," he added.
Scotland Yard issued a brief statement about the detention of Miranda, a Brazilian national, saying he’d been held briefly under a provision of an anti-terrorism law.
"At 08:05 on Sunday 18 August 2013,” said the statement, "a 28-year-old man was detained at Heathrow Airport under Schedule 7 of the Terrorism Act 2000. He was not arrested. He was subsequently released at 17:00.”
The White House on Monday directed questions about Miranda's detainment to the British government, saying that the decision to question him was made without the involvement or direction of the United States.
The British government did provide a "heads up" to American authorities that it planned to question Miranda, but "the United States was not involved in that decision or that action," said spokesman Josh Earnest.
"This was the British government, making a decision based on British law, on British soil about a British law enforcement action," Earnest said.
Carrie Dann of NBC News and Reuters contributed to this report.