A Buddhist monastery of of Thailand is the focus of a police investigation after 41 bodies were found at the site, which were likely used for meditation.

The bodies were found on Saturday in the monastery Pa Nakhon Chibobornin central Thailand’s Pisit province, a police officer said. There were death and donation certificates for each, he added, clarifying that no prosecutions have been brought for now.

The police want to verify, by contacting the families, that they voluntarily donated the body of their relative. “We are trying to confirm that no body was stolen,” noted the police officer who asked not to be named.

The investigation began after police discovered a dozen more bodies last Wednesday at another monastery in neighboring Kampaeng Phet province.

The head of the Pisit province monastery, Pra Ajarn Saiphon Pandito, told PBS television that the corpses were being used in a “meditation technique” noting that he “doesn’t know” how many people have adopted the technique he teaches. He described worshipers “meditating” in places where coffins containing human remains are kept.

Pisit police said they are working with authorities in other provinces to ascertain whether the practice is widespread.

This method of meditation with corpses appeared in Thailand in the 18th century and was widespread until the 20th, when it began to decline.