But if Tsarnaev is indeed executed, he will be the exception, not the rule. At least 14 times since 1993, federal prosecutors have sought the death penalty for an accused terrorist, as they are doing now. In many of those cases, as in this one, the crime involved was horrific and nationally publicized.

But only once was the defendant actually executed: Timothy McVeigh, the Oklahoma City bomber, in 2001.

There are a few reasons why this is true. For starters, terrorism cases are handled at the federal level, where the death penalty is much less common. Since 1976, only three people have been executed under federal law; at the state level, the toll is over 1,300.