Miranda said he was seized almost as soon as his plane landed at Heathrow. "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," he said.
Agents confiscated Miranda's computer, external hard drive, cellphone, DVDs, memory sticks and some paper documents.
In London, a British lawmaker called for police to explain why Miranda was 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.
Keith Vaz, chairman of Parliament's Home Affairs Select Committee, told the BBC that "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.
The case drew the ire of watchdog groups.
"It's incredible that Miranda was considered to be a terrorist suspect," said David Mepham, the British director at Human Rights Watch. "On the contrary, his detention looks intended to intimidate Greenwald and other journalists who report on surveillance abuses."
Britain's laws are not unique. U.S. customs officials can search the electronic devices of anyone entering the U.S. without a search warrant. According to a 2011 internal Homeland Security Department report, officers at the border can search the devices and in some cases hold on to them for weeks or months. The DHS has said such searches help law enforcement detect child pornographers or terrorists.