 They noted that the statute of limitations was necessary to prevent useless prosecutions, because after dozens of years the testimony of witnesses was often not reliable enough to secure a conviction.In the opinion of the federal authorities, imprescriptibility should remain reserved for crimes of exceptional gravity such as genocide and terrorist acts.Finally, the federal authorities argued that a previous change in law had already accomplished much of what the proponents desired, fixing the period of prescription at 15 years starting with the 18th birthday of the victim.