Spoiler Alert - Click to Toggle
The two guys in the control room can manipulate everything right down to the mood of the subjects, and they also control the appearance of the creatures and can unleash them from their cubicles at will (clearly demonstrated before the appearance of the first zombie). Would you place a wager at a roulette table that you know for a fact was rigged? Where is the element of chance when things can be made to occur on cue?