Nigerian police faced accusations on Friday of misleading the public about the use of a building in which the remains of 20 people were found this week.
Security officials are continuing their investigations into the case, after the discovery of the mummified bodies in a building they said was suspected of holding a ritual shrine in southern Nigeria's Edo state, the BBC reported.
But a civil society group published a statement on Friday saying the building was a newly built morgue used to "house evacuated corpses from an old facility".
It follows the arrest of thee suspects after police found the bodies.
Armed officers raided the building located in Benin city after receiving intelligence reports of a “suspected ritual shrine" and unusual activities in the location.
Authorities have not said how long the bodies had been in the house.
The building is located about five kilometres from the city centre.
“Fifteen mummified male corpses, three mummified female corpses and two mummified children corpses were discovered at the scene,” said Jennifer Iwegbu, a police spokeswoman.
Those arrested are young males and the police are intensifying efforts to arrest other fleeing suspects, Ms Iwegbu said.
A police medical team is also part of the investigation.
Ms Iwegbu told the public to "remain calm as authorities are continuing to ensure the safety of all law-abiding people in Edo."
Many residents arrived at the building expressing horror over what seemed to have been an operation hidden away from the eyes of even nearby neighbours.
In the past, Nigeria's security officials have uncovered such shrines used by ritualists and kidnappers, but considering the large number of victims the case is considered to be one of the most shocking in recent years.