The US government has said British officials gave it a "heads up" about the detention of the partner of a journalist who published information from US whistleblower Edward Snowden.
But it insisted it was not involved in the decision to detain David Miranda.
It said it was a matter for British law enforcement officials and the decision was taken "independent of our direction".
Mr Miranda, 28, was held for nine hours by police at Heathrow on Sunday.
He was detained under schedule 7 of the Terrorism Act 2000. This allows police to hold someone at an airport, port or international rail station for up to nine hours for questioning about whether they have been involved with acts of terrorism.