I know that when police in my area need to destroy evidence that's no longer needed (and can't just be disposed of in normal waste streams, or sold or what have you) they normally take it to a local garbage incineration plant.

There was also a steel mill in the area at one point and their furnace was occasionally been put to use for similar purposes (tangential - there was at least one instance I've heard of where the US mint used that furnace to dispose of a batch of coins they were testing a new alloy or process or something on)

Maybe in the area of Montana they were limited in choices, thought this place would do, but did not do a test run to ensure ventilation, leading to terrible results. Perhaps the unit was seldom used and poorly maintained (the fact that there were kittens in the same room as the incinerator seems crazy to me)

Read the article:

The incinerator is usually used by animal control officers to dispose of euthanised animals, but local authorities said it can also be used by law enforcement to burn seized narcotics.