David Miranda, the partner of Guardian journalist Glenn Greenwald, will take legal action against the United Kingdom over his nine-hour detention Sunday, his lawyers said Tuesday morning.

Miranda will argue that the police’s confiscation of his laptop, USB flash drives and other items was illegal, and that he wants them returned.

“We are concerned about the unlawful way in which these powers were used and the chilling effect this will have on freedom of expression,” said Kate Goold, a lawyer with the firm representing Miranda.

Miranda was detained by police at Heathrow Airport in London as he was about to fly home to Brazil, where he lives with Greenwald. UK police detained him for nearly nine hours, the maximum amount of time allowed by a provision of the UK’s Terrorism Act known as Section 7.

Miranda’s partner, Greenwald, is responsible for breaking the story of domestic surveillance programs carried out by the U.S. National Security Agency as well as the UK’s Government Communications Headquarters. He broke the story after being supplied with classified material by former contractor Edward Snowden.

The Guardian has said that it is not directly participating in the lawsuit, but supports Miranda’s action.