The bodies were exhumed for inquest on Monday in the presence of a magistrate, two doctors and police officers, police said.
The bodies belong to two families whose members were allegedly tortured and lynched by the villagers on suspicion that they practised witchcraft, the official said.
The incident took place in the first week of January this year in Mrigre village which is about 5 km away from Rongram and the matter was hushed up until police cracked the case while investigating a missing person's report made by a relative of one of the deceased, the official said.
A thorough investigation was launched into the case and statements of at least sixty villagers were recorded before the police were led to the two sites where the bodies were buried, he said.
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All the victims were found with their hands tied behind their backs and their bodies bore marks and patches indicating they suffered fatal injuries caused by blunt objects, the police official said.
In another grave, the bodies of Hallen T Sangma, his wife Bala T Sangma and their son Washing T Sangma (30) were found also with a similar kind of treatment meted out to them before they died, Singh said.
While a case of murder was registered, the SP said no one was arrested so far.