London police detained David Miranda, the partner of reporter Glenn Greenwald, under anti-terror legislation at the London airport on Sunday. Miranda arrived Monday in Rio de Janeiro, where he lives with Greenwald.
A defiant Greenwald promised that he was going "to write much more aggressively than before" about government snooping.
"I'm going to publish many more things about England, as well," he said. "I have many documents about the system of espionage of England, and now my focus will be there, too. I think they'll regret what they've done."
Miranda told reporters in Rio that he was not threatened while he was detained at Heathrow, but confirmed that personal objects were taken from him.
"I stayed in a room, there were six different agents, entering and leaving, who spoke with me," he said after being greeted by Greenwald with a hug. "They asked questions about my whole life, about everything. They took my computer, video game, cellphone, memory thumb drives, everything."
In London, a British lawmaker called for police to explain why Miranda had been detained — and why it took nearly nine hours to question him.
Miranda was held for nearly the maximum time that British authorities are allowed to detain individuals under the Terrorism Act's Schedule 7, which authorizes security agencies to stop and question people at borders.
"What needs to happen pretty rapidly is we need to establish the full facts," Keith Vaz, chairman of Parliament's Home Affairs Select Committee, told the BBC.
"Now you have a complaint from Mr. Greenwald and the Brazilian government — they indeed have said they are concerned at the use of terrorism legislation for something that does not appear to relate to terrorism. So it needs to be clarified, and clarified quickly."
Vaz said it was "extraordinary" that police knew that Miranda was Greenwald's partner and that the authorities were targeting partners of people involved in Snowden's disclosures.
Greenwald has written about NSA surveillance programs based on files disclosed by Snowden, who now has temporary asylum in Russia. The Obama administration wants Snowden sent back to the United States to be tried for the leaks.
Miranda, 28, was stopped while traveling home to Brazil after visiting Germany, where he met with Laura Poitras, a U.S. filmmaker who has worked with Greenwald on the NSA story.
British police acknowledged that they had detained a 28-year-old man at 8:05 a.m. He was released at 5 p.m. without being arrested, the Metropolitan Police Service said. They have not commented further.
The Guardian newspaper reported it paid for Miranda's flights but said that he was not an employee of the newspaper.
"As Glenn Greenwald's partner, he often assists him in his work," the newspaper said in statement. "We would normally reimburse the expenses of someone aiding a reporter in such circumstances."
In an email Monday to The Associated Press, Greenwald said he needed material from Poitras for stories he was working on with her relating to the NSA, and that he had things that she needed.
"David, since he was in Berlin, helped with that exchange," Greenwald wrote.
The Home Office defended Schedule 7 in a report last year, arguing it was designed to help authorities determine whether people crossing U.K. borders have been involved in the "commission, preparation or instigation of acts of terrorism." Border agents are not required to have reasonable suspicion before detaining a traveler.
The Home Office did not comment on Miranda's detention.
David Anderson, Britain's official independent reviewer of terrorism legislation, said he had asked authorities to explain why Miranda was detained and held for so long. He said fewer than 40 people out of 69,000 stopped under Schedule 7 in 2011-2012 were held for six hours or more.
In most cases, those questioned under Schedule 7 are detained for less than an hour.
"It is such a wide power that it would be surprising if it was used perfectly on every occasion," Anderson told the BBC. "It is a very extensive power and this just points up the need to have it properly controlled."
Isabella Sankey, policy director at the human rights organization Liberty, said the group had challenged Schedule 7 at the European Court of Human Rights. The British government is expected to submit submissions on the merits of the case in mid-September.
"This case very clearly highlights all the huge problems with this power," she said. "Shocking that it is, we're not surprised. We see this power used routinely. ... It's caused huge alienation."
A spokesman for Prime Minister David Cameron said that "Schedule 7 forms an essential part of the U.K.'s border security arrangements," but added that it was for the police to decide "when it is necessary and proportionate to use these powers."
Brooks reported from Rio de Janeiro.
Copyright 2013 The Associated Press.