The AMS is an international organization. It appears to deal with zombie-related infestations, as shown in The House of the Dead: Overkill and after the Curien incident. Its true purpose is never stated, however, but it appears to be outside of other US Government organizations and/or jurisdiction (for example, when G confronts Papa Caesar, he specifically places him under "AMS Arrest", though this may just be an example of the purposely bad writing).
By 2019, following a world-wide outbreak of zombies, it is obvious AMS is no more. However, former agent Thomas Rogan formes a paramilitary group combat the undead threat that roams the world.