Where did the saying your fats in the fire come from?

Many idioms debate from unconfirmed originations. This may be one of those, but I do know where law enforcement (specifically SWAT) base it off of. My father, a former FBI SWAT operative would work with a training program that works like a video game. It was called a FATS machine: Fire Arms Training ScenariOS. It worked pretty much like "Duck Hunt" in Nintendo, where the user would hold a fake firearm that would interact with the screen. The user would have to make the proper decisions and aim to pass the exercise. The pretend firearm would even have a C02 mechanism to replicate the recoil of a live firearm.

SWAT operatives recognize the FATS machine, and in live situations, FATS in the fire meant: "The FATS machine is real this time".

But like I said, it is only one possible origination of the idiom.