In California there is sort of a statute of limitation if the death is not immediate.
“194. To make the killing either murder or manslaughter, it is not requisite that the party die within three years and a day after the stroke received or the cause of death administered. If death occurs beyond the time of three years and a day, there shall be a rebuttable presumption that the killing was not criminal. The prosecution shall bear the burden of overcoming this presumption. In the computation of time, the whole of the day on which the act was done shall be reckoned the first.”
This is at the state level though. Apparently the federal level is variable if you are Sarah Brady and you lose your stage prop.