Police reportedly have found dozens of skulls, bones and even a fetus in a glass jar next to a bizarre-looking altar inside a building believed to be frequented by drug traffickers in Mexico City.

The unusual discovery was made at a property in Tepito last Tuesday and coincided with the arrests of 31 people on suspicion of drug cartel activity, according to Reuters.

Investigators combing through the building are said to have found more than 40 skulls clustered around a cross that was adorned with a face mask.

Other images of the display published by local media showed a wall of painted symbols – one of which was a goat head with hexagram around its horns, Reuters reported.

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The Mexico City Attorney General’s office says it is still working to figure out where the skulls came from and whether or not the recovered fetus is human.

Investigators also found 40 jawbones and 30 leg and arm bones inside the building, the attorney general's office added.