Ueslei Marcelino / Reuters file – Glenn Greenwald, the British journalist who first published the documents leaked by former NSA contractor Edward Snowden, testifies before a Brazilian congressional committee on NSA’s surveillance programs on Aug. 6.

By Michael Isikoff – NBC News National Investigative Correspondent

The partner of Glenn Greenwald — the journalist who first revealed classified documents provided by ex-National Security Agency contractor Edward Snowden — was detained by British security officials and questioned for nearly nine hours at Heathrow Airport outside London Sunday.

Greenwald, who writes for the U.K.-based Guardian newspaper, told NBC News that he was called early Sunday morning by a British security official and informed that authorities had detained David Miranda, with whom he lives in Rio de Janeiro, Brazil, under a provision of a British anti-terrorism law that permits authorities to stop, search, question and detain individuals at airports, ports and border areas.

“They questioned [Miranda] about my journalism,” Greenwald said.