(Capcom's primary storyline)
Heaven's Gate was the code call for a strategic military installation involved in the t-Virus epidemic of September 1998, which enacted "Mission Code XX", decimating Raccoon City in an attempt to contain the outbreak. Whilst the command and control center could feasibly be located anywhere within the United States or further, it is likely, by geography, that "Angel" flight was dispatched from Scott Air Force Base.
On October 1, 1998, "Mission Code XX", was greenlit by governmental order, following the deaths of all military personnel sent in to attempt to pacify the crisis and evacuate civilians. It was the final, desperate attempt to contain the outbreak, of which the military personnel involved were clearly uncomfortable with enacting. At least six strike fighters codenamed "Angel", were deployed to ensure Raccoon City's destruction. It is not known what happened to Heaven's Gate after the completion of Sterilization Operation.