Over 40 skulls have been found from the den of a drug cartel in Mexico City during a police raid this week.
The police also found dozens of bones and a fetus inside a jar placed next to an altar at the den, where 31 people were arrested on Tuesday for suspected drug activity.
Much to public criticism, 27 of those suspects were released.
The den is located in the Tepito neighborhood, a hotbed of illicit commercial activity.
Four skulls were built into the altar. Other skulls were clustered around the altar, that lay in front of a cross with a horned wooden face mask.
Further, authorities found several symbols of a pyramid topped with a hand and a goat's head with a hexagram between its horns.
Overall, at least 42 skulls have been found and authorities are still investigating the origins of the skulls, a spokesperson for the attorney general's office told Reuters.
The spokesperson said that the investigators have found knives, 40 jawbones, the fetus and 30 leg or arm bones at the site.
It wasn't immediately clear if the fetus was a human, the spokesperson added.
According to Mirror, cartel mobsters used the "Satanic" altar to make themselves "immune to bullets". The report added that the police allegedly found skulls covered in blood and several references to devil worshipping at the site of the drug cartel bust.
Security forces had nabbed the 31 suspects after authorities seized two laboratories which were used to make synthetic drugs, 50 kg of chemical precursors, more than two tons of marijuana and 20 kg of cocaine.
An unspecified amount of money, rocket launchers and grenades were also seized.
However, on Friday, President Andres Manuel Lopez Obrador said that 27 suspects had been released.
Speaking at a press conference, Lopez Obrador said, "Here the important thing is to see what the arguments were that were used to release these people."
He cautioned people against rushing to a judgment saying, "Let's not rush. If someone acted improperly, illegally, if there was corruption, we will condemn it."
The case has been seen as the second high-profile failure of his government.
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