The International Society for Civil Liberties and the Rule of Law said its findings showed the bodies were those of illegally arrested members of the public, robbery and kidnapping suspects, and members of the Movement for the Actualization of the Sovereign State of Biafra, MASSOB.
The victims were executed without investigation, prosecution, and sentencing, the non-governmental organization, which has carried out several reports and evaluation of police abuses, said.
The unidentified bodies were first found on January 19 as villagers of Amansea, the community near the river, went early morning to fetch water from the community's only drinking source.
The horror has shocked the nation and the world, but surprisingly the federal government has yet to make a pronouncement on the case.
While the villagers said the bodies numbered more than 30, police insists they were less than 20.
MASSOB had earlier claimed the bodies were those of its members who went missing months after police took them into custody. Police has denied the claim.
The International Society for Civil Liberties and the Rule of Law said it spoke with several relatives of missing family members, and community sources which pointed at the SARS, a deadly unit of the police often accused of abuses.
"What are the whereabouts of the nine MASSOB activists transferred to SARS from the Onitsha Area Command since 8th day of December, 2012? Were they granted interim bail? If yes, who were their sureties and where are the records with which they were granted bail," the group asked in a report signed by its chairman, Emeka Umeagbalasi.
"Were the detainees arraigned in either courts of inferior records or in courts of superior records? If yes, which divisions of the courts in Anambra State and at what dates?
"What was their fate after the courts' charges or arraignments? Were they granted bails or remanded in prisons' custodies? If yes who were their sureties or which prisons or police stations were they remanded and by which courts and dates? Are they still being detained by SARS without being charged to, or arraigned in courts? If yes, which of the SARS units are they being detained?"
The group called on the federal government to make a categorical statement on the case, and order investigation while ensuring payment of compensation to the families involved.
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