By Pedro Fonseca

RIO DE JANEIRO (Reuters) – The journalist who first published secrets leaked by fugitive former U.S. intelligence agency contractor Edward Snowden vowed on Monday to publish more documents and said Britain will be “sorry” for detaining his partner for nine hours.

British authorities used anti-terrorism laws on Sunday to detain David Miranda, partner of U.S. journalist Glenn Greenwald, as he passed through London’s Heathrow airport.

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Miranda, 28, a Brazilian citizen, said he was questioned for nine hours before being released without charge, minus his laptop, cellphone and memory sticks, which were seized.

Greenwald, a columnist for Britain’s the Guardian newspaper who is based in Rio de Janeiro, said the detention was an attempt to intimidate him for publishing documents leaked by Snowden disclosing U.S. surveillance of global internet communications.

Snowden, who has been granted asylum by Russia, gave Greenwald from 15,000 to 20,000 documents with details of the U.S. National Security Agency’s surveillance programs.

“I will be far more aggressive in my reporting from now. I am going to publish many more documents. I am going to publish things on England too. I have many documents on England’s spy system. I think they will be sorry for what they did,” Greenwald, speaking in Portuguese, told reporters at Rio’s airport where he met Miranda upon his return to Brazil.

“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.

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Miranda told reporters that six British agents questioned him continuously about all aspects of his life during his detention in a room at Heathrow airport. He said he was freed and returned his passport only when he started shouting in the airport lounge.

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.

Many Brazilians are still upset with Britain’s anti-terrorism policies because of the death of Brazilian electrician Jean Charles de Menezes, who was mistaken for a suspect in a bombing attempt in 2005. Menezes was shot seven times in the head by police on board an underground train at a London station.

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Greenwald met with Snowden in June in Hong Kong, from where he published the first of many reports that rattled the U.S. intelligence community by disclosing the breadth and depth of surveillance by the NSA on telephone and internet communications.

Snowden faces criminal charges in the United States after leaking documents disclosing the previously secret U.S. internet and telephone surveillance programs. Russia rejected American pleas to send Snowden back to the United States for trial, instead granting him a year’s asylum on August 1.

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Brazil, whose president, Dilma Rousseff, is scheduled to make a state visit to Washington in October, declined to consider an asylum request from Snowden. But some politicians angered by the disclosure of NSA surveillance of internet communications of Brazilians proposed granting him asylum in Brazil.

(Reporting by Pedro Fonseca; Writing by Anthony Boadle; Editing by Brian Winter and Eric Beech)