BOSTON — Since the federal death penalty was reinstated, in 1988, attorneys general have authorized it for about 500 defendants. By the end of the month there may be yet another: the accused Boston Marathon bomber, Dzhokhar Tsarnaev.

Attorney General Eric H. Holder Jr. must decide by Jan. 31 whether to pursue the death penalty, but even if he does so, it is far from certain that Mr. Tsarnaev would actually face execution. Of those 500 defendants, only three have been executed, the last one a decade ago, according to the Federal Death Penalty Resource Counsel.

Still, Mr. Holder’s job is not to weigh the probabilities of Mr. Tsarnaev’s execution. Instead, he must decide whether the aggravating factors that might justify death in this case, like the indiscriminate killing and maiming of innocent people, outweigh any mitigating factors, such as the possibility that Mr. Tsarnaev, who was 19 at the time, was under the sway of his older brother.

While Mr. Holder has said he does not personally support the death penalty, he has authorized its use several times, and many legal experts expect he will do so again in this case.