The 'terror watch list' isn't regularly watched, and it's not one list.
Reports that Tamerlan Tsarnaev had been added to the government's terror watch list seems to bolster the argument that the marathon bombing is the result of FBI error. But that revisionism fails to take into account the scale and complexity of how the government tracks terror suspects.
The terror watch list, as it's known, isn't really a watch list. For one thing, it isn't regularly watched. For another, it's not one list. It's more of a set of hierarchical, integrated databases which are checked under various circumstances, most notably when individuals want to travel. According to Reuters, after he was interviewed by the FBI in 2011, Tsarnaev was added to the Terrorist Identities Datamart Environment, or TIDE, which is compiled by the National Counterterrorism Center. It's a list that comprises over half a million names. "Because of its huge size," Reuters reports, "U.S. investigators do not routinely monitor everyone registered there, said U.S. officials familiar with the database."
In other words, there's a sort of pyramid of terror investigation. At the bottom of the pyramid are hundreds of thousands of people who've come to the government's attention for some reason. As the FBI and other agencies look into behavior and patterns, people can move up the pyramid — fewer people evincing more suspicious behavior — winnowing to a point once held by Osama bin Laden. Or, after a determined time, people can drop out of the pyramid entirely if they don't behave in a way that raises suspicion. That's the track Tsarnaev was on.