Glenn Greenwald's partner David Miranda was on his way home to Brazil on Sunday when UK officials, citing a controversial terrorism law, detained him for 9 hours at London's Heathrow airport. The Guardian's report explains that Miranda was held for the maximum amount of time allowed by law under a provision, applicable only to airports and other border areas, that permits authorities to detain, search, and question individuals. During that time, according to Greenwald, Miranda was question about the Guardian's reports on NSA data collection from whistleblower Edward Snowden.

Officials took Miranda's laptop, camera, memory sticks, game consoles, and DVDs before letting him go without charge. The Guardian reports that the paper is "urgently" seeking clarification from British officials on the reason for Miranda's detainment. But Greenwald, based on what he's learned so far of the details of his partner's detainment, has a pretty solid idea of why it happened:

While in Berlin, Miranda had visited Laura Poitras, the US film-maker who has also been working on the Snowden files with Greenwald and the Guardian.

"This is a profound attack on press freedoms and the news gathering process," said Greenwald. "To detain my partner for a full nine hours while denying him a lawyer, and then seize large amounts of his possessions, is clearly intended to send a message of intimidation to those of us who have been reporting on the NSA and GCHQ. The actions of the UK pose a serious threat to journalists everywhere."

In a separate piece, Greenwald explains more of how he learned that his partner was detained:

At the time the "security official" called me, David had been detained for 3 hours. The security official told me that they had the right to detain him for up to 9 hours in order to question him, at which point they could either arrest and charge him or ask a court to extend the question time. The official - who refused to give his name but would only identify himself by his number: 203654 - said David was not allowed to have a lawyer present, nor would they allow me to talk to him.

Officials spent their time questioning Miranda about the reporting Greenwald and Poitras had done on the NSA files released by Edward Snowden, and wanted to know the content of the electronic material he was carrying, according to Greenwald. Citing the UK's own explanation of the law under which his partner was detained (Schedule 7 of the 2000 Terrorism Act), Greenwald notes that it's almost unheard of for someone to be held for the full 9 hours allowed by law — 97 percent of detentions last under an hour, for instance. The law allows officials to question individuals in selected border and transit areas "to determine whether that person is or has been involved in the commission, preparation or instigation of acts of terrorism."

Update: The New York Times confirms that Miranda's trip, as implied (but not overtly stated) by the Guardian's report that Miranda was visiting journalist Laura Poitras in Germany, was connected to Greenwald's reporting. His flights were paid for by the Guardian, they add, citing an interview with Greenwald.

Update 7:15 p.m.: Here the Guardian's front page tomorrow.