Statutes of Limitation in Federal Criminal Cases: An Overview

 "A statute of limitations dictates the time period within which a legal proceeding must begin. The purpose of a statute of limitations in a criminal case is to ensure the prompt prosecution of criminal charges and thereby spare the accused of the burden of having to defend against stale charges after memories may have faded or evidence is lost..."
https://www.fas.org/sgp/crs/misc/RL31253.pdf