User blog comment:Argorrath/Treyarch releases a BO2 teaser video called "Origins" for DLC 4/@comment-9261790-20130811170927/@comment-3171236-20130811172003

Assassin: The level discription of Der Riese specifically states "This is where it all began." There are recordings throughout the map that also show experimentation with the zombies, and the creation of the Hellhound.

Onyx: The 4 are never implied to have met in Shi No Numa. The original trailer even has dialogue showing their arrival to the facility (with Nikolai claiming "Good, I was getting tired of Takeo's rambling").