Mr. Greenwald said he received a call early on Sunday from someone who identified himself as a security official from Heathrow Airport and who informed him that Mr. Miranda had been detained, at that point for three hours. The British authorities, he said, told Mr. Miranda that they would obtain permission from a judge to arrest him for 48 hours, but he was released at the end of the nine hours, around 1 p.m. Eastern time.
Mr. Miranda was in Berlin to deliver documents related to Mr. Greenwald’s investigation into government surveillance to Ms. Poitras, Mr. Greenwald said. Ms. Poitras, in turn, gave Mr. Miranda different documents to pass to Mr. Greenwald. Those documents, which were stored on encrypted thumb drives, were confiscated by airport security, Mr. Greenwald said. All of the documents came from the trove of materials provided to the two journalists by Mr. Snowden. The British authorities seized all of his electronic media — including video games, DVDs and data storage devices — and did not return them, Mr. Greenwald said.It seems to me that the authorities were probably concerned that Miranda had in his possession sensitive materials related to the Snowden leaks, which Miranda's partner has previous indicated he has access to and has also stated that he might put in the hands of Mr. Miranda. I don't think that that makes a 9 hour detention right, necessarily, because it does more to give Greenwald something to report about regarding government-fucking-overreach than it does to actually stop any sensitive information from being spread. If anything, it proves his point regarding difficulties faced by journalists trying to do legitimate work, because this detention is associated with a statute regarding terrorism--and I think Miranda is hardly to be considered on a par with bombers and hostage-takers.
I do think the British government believed that they had reasons, and I wouldn't be surprised if the detention would be considered lawful. The problem is what even very civilized people have come to accept as lawful that is the problem.
At the same time he very well probably did have something on him they were interested in. I'm not sure that proves their point, though.