There's been a lot of controversy this week over the revelation that the Obama administration has been collecting Verizon customers' phone data, but it appears at least one terrorist plot was cracked thanks to email surveillance; according to court documents, the National Security Administration thwarted a 2009 terrorist plot to bomb New York City subways after reading an email sent to an Al-Qaida leader.

The email in question was reportedly sent by Najibullah Zazi, a former airport shuttle driver who pleaded guilty in 2009 to plotting to blow up the subways. Apparently, his plan was foiled thanks to a program called PRISM, which is tasked with spying on non-US citizens' online activity. Zazi sent an email to a rarely-used Yahoo address tthat British Intelligence recognized as belonging to an Al-Qaida leader; finding that suspicious, both the British government and US intelligence tracked down Zazi's IP address in Aurora, Colorado, followed him as he drove to New York and stopped him before he could follow through with his plan.

In a statement this week, Director of National Intelligence James Clapper explained the limitations of the US's phone and data surveillance, and noted that the government can only review such data "when there is a reasonable suspicion, based on specific facts, that the particular basis for the query is associated with a foreign terrorist organization." And a member of the House Intelligence Committee noted this week that prior surveillance had helped thwart a terrorist activity. Zazi has not yet been sentenced for trying to carry out the plot.