It happened back in the late 1930s, right before World War 2. As it so happens, one of the Masters involved decided to start killing potential masters before the grail war began, in an effort to thin out the herd of potential masters. Their thought being that, with weaker masters as opponents, they could ensure victory.

Unfortunately for them, their efforts didn't go unnoticed. A senior detective and his trainee managed to figure out that this person was murdering others, and the two went and tried to capture them. But the trainee was, unknowingly, the lost child of a magus family, and thus had the talent needed to summon a servant. Which he subsequently did, at the insistence of his senpai.