FROM NEW YORK CITY’S subways to Baltimore’s highway tunnels, October brought repeated instances of local authorities stepping up security in response to terrorist threat warnings. After New York City police decided to scale back their heightened security measures on October 10–just four days after the subway threat was announced–second-guessing about Mayor Bloomberg’s decision to raise the alert kicked into high gear. The threat to Baltimore’s Harbor Tunnel and Fort McHenry Tunnel had a similar ending, as city officials were criticized for closing both. New York City and Baltimore officials presented almost identical defenses, arguing that it’s best to err on the side of caution. Framed in this way, their actions seem unassailable: Mistakenly raising the terror alert only wastes resources, while mistakenly deciding not to raise it can cost lives. But is it really this simple?

In fact, captured terrorists have for some time put forward false information about supposed plots in progress. The New York and Baltimore cases are only the most recent examples of how an overseas source can provide an uncorroborated story that triggers a potentially disproportionate reaction in the United States.