David Miranda, journalist Glenn Greenwald's partner, was in a U.K. court Wednesday challenging the legality of his nine-hour detention and the confiscation of material by police at London's Heathrow Airport under antiterror laws in August.
Miranda was detained and questioned Aug. 18 by British authorities while in transit from Berlin to Rio de Janeiro, where he lives with Greenwald, the former Guardian journalist who broke the story of National Security Agency (NSA) surveillance with materials provided by whistle-blower Edward Snowden.
Although Miranda was not arrested or charged with any crimes, British authorities confiscated materials he was carrying, including electronic media that contained 58,000 documents from the NSA and its British counterpart, the Government Communications Headquarters (GCHQ).
Miranda filed a legal action against the British government soon after his release, seeking a judicial review of the legality of his detention as well as the return of his materials.
In day one of a hearing expected to last two days, attorneys argued that he should have been protected as a journalist, The Guardian reported.
At issue is whether Miranda was properly detained under the U.K.'s antiterrorism rule, known as Schedule 7, regarding stopping and seizure at border crossings.
British authorities argued that Miranda was involved in "terrorism" when he tried to carry classified documents from Snowden through the airport, according to police and intelligence documents provided to The Guardian.
Government lawyers argued that the purpose in stopping him was to "neutralize" the effects of "improper dissemination" of sensitive material that could have endangered lives.
At a preparatory court hearing last week for Miranda's lawsuit, new details of how and why British authorities decided to act against him were made public through materials including extracts from police and MI5 documents.
A document that was circulated to British border posts before Miranda's arrival, called a Ports Circulation Sheet, was read into the record. It was prepared by Scotland Yard in consultation with the MI5 counterintelligence agency.
"Intelligence indicates that Miranda is likely to be involved in espionage activity which has the potential to act against the interests of UK national security," the document stated.
"Additionally the disclosure, or threat of disclosure, is designed to influence a government and is made for the purpose of promoting a political or ideological cause. This therefore falls within the definition of terrorism."
In a separate document read into the court record, MI5 indicated that British authorities' interest in Miranda was spurred by his apparent role as a courier ferrying material from Laura Poitras, a Berlin-based filmmaker, to Greenwald in Brazil.
"We strongly assess that Miranda is carrying items which will assist in Greenwald releasing more of the NSA and GCHQ material we judge to be in Greenwald's possession," said the document, described as a National Security Justification prepared for police.
"Our main objectives against David Miranda are to understand the nature of any material he is carrying, mitigate the risks to national security that this material poses," the document added.
British authorities said in August they had opened a criminal investigation after initially examining materials they seized from Miranda. They did not spell out the probe's objectives.
Separately, last Friday, media disclosed details of an open letter Snowden had issued to Germany from his place of exile in Russia, in which he said his revelations have helped to "address formerly concealed abuses of the public trust" and added that "speaking the truth is not a crime."
Snowden said he was counting on international support to stop Washington's "persecution" of him for revealing the scale of its worldwide phone and Internet surveillance.
Al Jazeera and Reuters. Amel Ahmed contributed to this report.