LONDON -- Demands grew on Monday for the British government to explain why it had used antiterrorism powers to detain the partner of a journalist who has written about surveillance programs based on leaks by the former National Security Agency contractor Edward J. Snowden.
David Michael Miranda, a Brazilian citizen and the partner of the American journalist Glenn Greenwald, who lives in Brazil, was held Sunday at Heathrow Airport in London for nine hours, the maximum allowed by law, before being released without charge.
"They were threatening me all the time and saying I would be put in jail if I didn't cooperate," Mr. Miranda said Tuesday in an interview with The Guardian newspaper, where Mr. Greenwald is a columnist. "They treated me like I was a criminal or someone about to attack the U.K."
On its Web site, The Guardian said the interview was the first since Mr. Miranda returned to his home in Rio de Janeiro on Monday. "It was exhausting and frustrating, but I knew I wasn't doing anything wrong," Mr. Miranda said. Speaking separately on Monday, he said that all of his electronic equipment, including his laptop computer and cellphone, had been confiscated. In the interview, he added that he was not allowed to call his partner, who is a qualified lawyer in the United States, nor was he given an interpreter, despite being promised one because he felt uncomfortable speaking in a second language.
"I was in a different country with different laws, in a room with seven agents coming and going who kept asking me questions. I thought anything could happen. I thought I might be detained for a very long time," he said.
Mr. Miranda was traveling from Berlin to Rio de Janeiro. In Berlin, he had met with Laura Poitras, an American filmmaker who has worked with Mr. Greenwald on the Snowden leaks about secret American and British surveillance programs that they argue violate individual rights and liberties.
The Guardian, where Mr. Greenwald is a columnist, reported that it had paid for Mr. Miranda's flights but that he was not an employee of the paper. "As Glenn Greenwald's partner, he often assists him in his work," The Guardian said in statement. "We would normally reimburse the expenses of someone aiding a reporter in such circumstances."
In an e-mail Monday to The Associated Press, Mr. Greenwald said that he needed material from Ms. Poitras for articles he was working on related to the N.S.A., and that he had things she needed. "David, since he was in Berlin, helped with that exchange," Mr. Greenwald wrote.
Keith Vaz, an opposition Labour Party legislator who is chairman of Parliament's Home Affairs select committee, said he had written to the head of the Metropolitan Police Service, which has jurisdiction in the matter, to ask for clarification of what he called an extraordinary case.
"What needs to happen pretty rapidly is, we need to establish the full facts," he 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."
The editor of The Guardian, Alan Rusbridger, disclosed on Monday that the British government had sent officials from Government Communications Headquarters, which is known as GCHQ and is the British version of the National Security Agency, to the newspaper's offices in London to destroy computers containing documents leaked by Mr. Snowden. Mr. Rusbridger said that he had protested that the same information was available elsewhere, but that the officials had insisted on proceeding.
"And so one of the more bizarre moments in The Guardian's long history occurred -- with two GCHQ security experts overseeing the destruction of hard drives in The Guardian's basement just to make sure there was nothing in the mangled bits of metal which could possibly be of any interest to passing Chinese agents," he wrote, adding, "We will continue to do patient, painstaking reporting on the Snowden documents, we just won't do it in London."
The police said in a statement that Mr. Miranda, 28, had been lawfully detained under Schedule 7 of Britain's Terrorism Act 2000, which allows them to stop and question people traveling through ports and airports to determine whether they are involved in planning terrorist acts.
Mr. Miranda told The Guardian that, as his flight from Berlin approached London, "there was an announcement on the plane that everyone had to show their passports. The minute I stepped out of the plane they took me away to a small room with four chairs and a machine for taking fingerprints."
His carry-on bags were searched, The Guardian quoted Mr. Miranda as saying, and the police confiscated a computer, two thumb drives, an external hard drive and several other electronic items, including a games console, as well two newly bought watches and phones that were packaged and boxed in his checked luggage.
"They got me to tell them the passwords for my computer and mobile phone," Mr. Miranda said. "They said I was obliged to answer all their questions and used the words 'prison' and 'station' all the time."
"It is clear why they took me. It's because I'm Glenn's partner. Because I went to Berlin. Because Laura lives there. So they think I have a big connection. But I don't have a role. I don't look at documents. I don't even know if it was documents that I was carrying. It could have been for the movie that Laura is working on."
Mr. Vaz and his party said they wanted to know how the government could justify using Schedule 7 in this case, arguing that any suggestion that antiterrorism powers had been misused could undermine public support for those powers.
A Home Office spokesman said Monday that the detention was an operational police matter and that neither he nor the police would provide any details. "Schedule 7 forms an essential part of the U.K.'s security arrangements," the spokesman said. "It is for the police to decide when it is necessary and proportionate to use these powers."
David Anderson, Britain's official independent reviewer of terrorism legislation, said he had also asked the authorities to explain why Mr. Miranda was detained for so long. He said fewer than 40 of the 69,000 people stopped under Schedule 7 in 2011-12 were held for six hours or more. In most cases, Mr. Anderson said, 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," Mr. Anderson told the BBC. "It is a very extensive power, and this just points up the need to have it properly controlled."
A White House spokesman, Josh Earnest, told reporters Monday that the British government had given the United States notice that it intended to detain Mr. Miranda when his plane landed, but that there had been no American request to do so.
"This is the British government making a decision based on British law on British soil about a British law enforcement action," he said, adding, "This is something that they did not do at our direction, is not something that we were involved with. This is a decision that they made on their own."
He and other administration officials declined to say on Monday whether the British had given the United States government any of the electronic materials seized from Mr. Miranda.
Mr. Miranda arrived Monday in Rio de Janeiro and was greeted by Mr. Greenwald, who said that in response to the detention, he planned "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."
Mr. Miranda told reporters in Rio on Monday that all of the documents encrypted on the thumb drives came from the trove of materials provided by Mr. Snowden.
Nick Cohen, a columnist for the conservative weekly The Spectator, wrote on Monday that the detention of Mr. Miranda was "a clarifying moment that reveals how far Britain has changed for the worse."
Nearly everyone, Mr. Cohen wrote, suspects that the police held Mr. Miranda "on trumped-up charges because the police, at the behest of the Americans, wanted to intimidate Miranda's partner, Glenn Greenwald, the conduit of Edward Snowden's revelations, and find out whether more embarrassing information is on Greenwald's laptop."
He criticized the police for saying so little about the case, and concluded: "The next time they try to tell you that the secrecy and attempts to silence legitimate debate are 'in the public interest,' do not forget what they did to David Miranda, because they can do it to you, too."
Charlie Savage contributed reporting from Washington.
This article originally appeared in The New York Times.