Mexican authorities have found dozens of human skulls, bones and even a fetus in a glass jar next to an altar in the den of suspected drug traffickers.

Prosecutors in Mexico City said 42 skulls, 40 jaw bones and 31 “long” bones — probably from arms or legs — were discovered last week at a property in the neighborhood of Tepito, were 31 people were arrested on suspicion of drug cartel activity.

A judge later ordered 27 of the suspects released.

DNA tests were being conducted on the bones to determined their identities and origins.

Four of the skulls were built into the altar, which appeared to be of an Afro-Caribbean religion, officials said.

A macabre photo distributed by the Mexico City attorney general’s office shows the skulls clustered around the altar, which has a cross behind it adorned with a horned wooden face mask.

To the side of the altar is a painted wall full of symbols that include a pyramid topped with a hand, celestial bodies and the head of a goat with a hexagram between its horns.

Tepito, north of the capital’s historic center, has been known as a hotbed of illegal commercial activity.

Practitioners of at least one Afro-Caribbean religion have previously been charged with using human bones in their rituals, but Mexico’s own pre-Hispanic religion featured public displays of skulls hanging on wooden racks.

Mexico’s present-day “Santa Muerte” cult also worships a skeleton figure. It was unclear if the remains belong to gang victims or were stolen from graveyards.

Bones also sometimes emerge because of above-ground burial niches and the tradition of cleaning grave sites for reuse after a few decades.

There has been an illicit trade in human bones for years in the city — but a more sinister origin for such bones is possible: In 2018, a man in Ecatepec on the outskirts of Mexico City confessed to murdering at least nine women and claimed to have sold their bones to another man.