Since the 1998 Curien Mansion incident, AMS has fought several incidents involving hostile biologically-engineered creatures. In 2003, the agency disbanded in wake of several other incidents. All remaining AMS agents have continued to collaborate in order to protect mankind, with top-ranking agent Thomas Rogan leading a secret division of survivors to accomplish this.
While the AMS' true purpose is never stated, it appears to be outside of other US Government organizations and/or jurisdiction. For example, when G confronts Papa Caesar he specifically places Caesar under "AMS arrest", though it is left ambiguous whether G actually has this vested authority or if this is a simple threat. As revealed in The House of the Dead 4, the agency has a European branch office.
The Zombie Revenge instruction manual claims that the AMS is an American agency, this was later verified in Overkill, revealing that it was active during the 1950s being involved in the secrecy of the experimental "Formula X" that was originally intended to be used in "OPERATION OVERKILL".
The House of the Dead's Sega Saturn manual mentions an AMS Director, who claims to have some experience on Dunwich and the Miskatonic delta.
Overkill also mentioned an AMS Director as well, though it is unknown if they are the same individual.
It is currently unknown what AMS stands for. The House of the Dead development team asked series director Takashi Oda about this; he indicated that if they ever created a game "based on a time before [the original House of the Dead]", he would like to explain AMS' founding.
↑Kori (September 7, 2012). "Website of the Dead’s Exclusive Interview with Takashi Oda". The Website of the Dead. Archived from the original on April 19, 2020. “After 16 years of the first game’s release, many fans have been wondering this – what does AMS stand for? The development team asked me this, too. I told them that if we ever create a game based on a time before The House of the Dead 1, I would like to explain how AMS started.”