I believe this fic is a Criminal Minds crossover, but it may have been NCIS. Sometime after the season 7 finale, the crossover law enforcement agency is looking into the destruction of Dick Roman's business empire and find the drugs that were intended to sedate the human public via Biggerson's. They come to the conclusion that some sort of terrorist plot had been foiled, and I believe that they discovered that the real Dick Roman had been killed months prior and impersonated in order to further this plot. They believe that the Leviathans were terrorists using some sort of 3D-printed masks made of an unseen-before material, and that the shapeshifters that the Winchesters stopped in "Skin" and "Nightshifter" were actually test-runs for the terrorist group's 3D-printed masks. Crossover-LEOs want to interview Sam 'n Dean (and possibly other hunters, I don't recall) about Dick Roman, as the boys' "religious beliefs" in shapeshifters, Leviathan, etc., allowed them to realize that the "terrorists" were impersonators, causing the terrorists to impersonate them in order to discredit them and stop them from interfering. I don't remember whether the crossover characters come to realize that monsters are real.

Thanks in advance!