I have to admit this story has me puzzled. Why would the British do something like this? What did they hope to gain, and why did they think it worth the cost? And -- of course -- were the British acting on their own under the Official Secrets Act, or were they acting on behalf of the United States? (My initial assumption was that they were acting on behalf of the U.S., but after the bizarre story of the British GCHQ demanding the destruction of Guardian computers last month, I'm not sure anymore.)

We do know the British were waiting for Miranda. It's reasonable to assume they knew his itinerary, and had good reason to suspect that he was ferrying documents back and forth between Greenwald and Poitras. These documents could be source documents provided by Snowden, new documents that the two were working on either separately or together, or both. That being said, it's inconceivable that the memory sticks would contain the only copies of these documents. Poitras retained copies of everything she gave Miranda. So the British authorities couldn't possibly destroy the documents; the best they could hope for is that they would be able to read them.

Is it truly possible that the NSA doesn't already know what Snowden has? They claim they don't, but after Snowden's name became public, the NSA would have conducted the mother of all audits. It would try to figure out what computer systems Snowden had access to, and therefore what documents he could have accessed. Hopefully, the audit information would give more detail, such as which documents he downloaded. I have a hard time believing that its internal auditing systems would be so bad that it wouldn't be able to discover this.

So if the NSA knows what Snowden has, or what he could have, then the most it could learn from the USB sticks is what Greenwald and Poitras are currently working on, or thinking about working on. But presumably the things the two of them are working on are the things they're going to publish next. Did the intelligence agencies really do all this simply for a few weeks' heads-up on what was coming? Given how ham-handedly the NSA has handled PR as each document was exposed, it seems implausible that it wanted advance knowledge so it could work on a response. It's been two months since the first Snowden revelation, and it still doesn't have a decent PR story.

Furthermore, the U.K. authorities must have known that the data would be encrypted. Greenwald might have been a crypto newbie at the start of the Snowden affair, but Poitras is known to be good at security. The two have been communicating securely by e-mail when they do communicate. Maybe the U.K. authorities thought there was a good chance that one of them would make a security mistake, or that Miranda would be carrying paper documents.

Another possibility is that this was just intimidation. If so, it's misguided. Anyone who regularly reads Greenwald could have told them that he would not have been intimidated -- and, in fact, he expressed the exact opposite sentiment -- and anyone who follows Poitras knows that she is even more strident in her views. Going after the loved ones of state enemies is a typically thuggish tactic, but it's not a very good one in this case. The Snowden documents will get released. There's no way to put this cat back in the bag, not even by killing the principal players.