The British Government gave the United States advance notice that London police intended to detain the partner of an American journalist who has written articles based on leaks from former National Security Agency contractor Edward Snowden, the White House says.
But the US did not request the detention and was not involved in the decision, a White House spokesman said.
Police detained David Miranda, the partner of reporter Glenn Greenwald, under anti-terror legislation at Heathrow airport on Sunday.
He was being held and has been questioned for nearly nine hours. Mr Miranda said he was not threatened while detained but said personal objects were taken from him, including his computer, mobile phone and digital memory sticks.
Mr Greenwald promised he was going "to write much more aggressively than before" about government snooping.
"I'm going to publish many more things about England, as well," he said in Portuguese at Rio's international airport when Mr Miranda arrived. "I have many documents about England's espionage system, and now my focus will be there, too. I think they'll regret what they've done."
Mr Miranda is in a civil union with Mr Greenwald and the pair live in Rio.