(CNN) -- Glenn Greenwald, the reporter who broke the news about secret U.S. surveillance programs, said the authorities who took his partner into custody at London's Heathrow Airport "are going to regret what they did."
"I am going to write my stories a lot more aggressively now," the Guardian reporter told Brazil's Globo TV on Monday in Rio de Janeiro.
"I am going to publish many more documents now. I am going to publish a lot about England, too, I have a lot of documents about the espionage system in England. Now my focus is going to be that as well."
Greenwald's partner, 28-year-old David Miranda, was held for nearly nine hours. He was reportedly passing through the airport on his way home to Brazil after leaving Berlin. Authorities seized his laptop, phone, and other materials.
The White House knew the move was coming.
"There was a heads up that was provided by the British government," White House spokesman Josh Earnest said Monday.
So the United States knew it "was likely to occur, but it's not something that we've requested and it's something that was done specifically by the British law enforcement officials there," he said.
He would not comment on whether the United States has obtained material from Miranda's laptop -- and would not say whether President Barack Obama condemns the detention.
Agents asked 'about my entire life'
Miranda, also speaking to Globo TV in Rio, said agents were asking him questions "about my entire life."
"I was in a room, there were six different agents coming in and out and talking to me," he said. "They took my computer, video games, cell phone, everything."
The detention was reported by The Guardian. Before releasing him, authorities seized Miranda's laptop, cell phone, video game consoles and USB sticks, Greenwald wrote for The Guardian.
"This is obviously a rather profound escalation of their attacks on the news-gathering process and journalism," he said.
"It's bad enough to prosecute and imprison sources. It's worse still to imprison journalists who report the truth. But to start detaining the family members and loved ones of journalists is simply despotic. Even the Mafia had ethical rules against targeting the family members of people they felt threatened by."
A Metropolitan Police spokesman confirmed that a 28-year-old man was detained Sunday at Heathrow.
The spokesman said the man was held for close to nine hours under Schedule 7 of the Terrorism Act 2000.
Brazil's foreign ministry issued a statement Sunday expressing "grave concern" over the incident. Anger had erupted in Brazil when citizens learned of U.S. National Security Agency spying on Brazil.
"This measure is without justification since it involves an individual against whom there are no charges that can legitimate the use of that legislation. The Brazilian government expects that incidents such as the one that happened to the Brazilian citizen today do not repeat."
According to The Guardian, nine hours is the maximum time allowed before authorities must either release or arrest a detained individual.
Miranda was returning to their home in Rio de Janeiro.
While in Berlin, Miranda stayed with filmmaker Laura Poitras, who has worked "extensively" with Greenwald on his stories about the National Security Agency, the reporter wrote.
The Guardian reported that it paid for Miranda's flights. "Miranda is not a Guardian employee but often assists Greenwald in his work," the newspaper said.
Guardian editor Alan Rusbridger, in an editorial, wrote that "Miranda is not a journalist, but he still plays a valuable role in helping his partner do his journalistic work."
He added that Greenwald's work on the reams of material provided by Snowden has been "immensely complicated by the certainty that it would be highly unadvisable for Greenwald (or any other journalist) to regard any electronic means of communication as safe. The Guardian's work on the Snowden story has involved many individuals taking a huge number of flights in order to have face-to-face meetings."
Miranda is quoted by the Guardian as saying: "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."
"If the UK and U.S. governments believe that tactics like this are going to deter or intimidate us in any way from continuing to report aggressively on what these documents reveal, they are beyond deluded," said Greenwald.
"If anything, it will have only the opposite effect: to embolden us even further."