MEXICO CITY (Reuters) - Police found more than 40 skulls, dozens of bones and a fetus in a glass jar next to an altar in the den of suspected drug traffickers in Mexico City during a raid this week, authorities said on Sunday.

Four of the skulls were built into the altar in the central Tepito neighborhood, where police arrested 31 people on Tuesday on suspicion of drug cartel activity, the city government said in a statement. A judge ordered 27 of the suspects released.

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

To the right of the altar was a painted wall full of symbols that included a pyramid topped with a hand, celestial bodies and the head of a goat with a hexagram between its horns, according to photos of the room published by local media.

In front of the wall stood a variety of objects, including dozens of wooden sticks with colored markings.

A spokeswoman for the attorney general’s office said authorities were still investigating the provenance of the skulls, at least 42 of which had been found.

Investigators also discovered knives, 40 jawbones, the fetus and 30 leg or arm bones at the site, the office said. It was not yet clear whether the fetus was human, the spokeswoman said.

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

The release of nearly all the suspects arrested in the raid on the warren of hidden tunnels and suspected drug laboratories in Tepito was seen as a setback to the government, which has been struggling to get a grip on chronic gang violence.