Nigeria has largely failed to investigate hundreds of killings by police and relatives of the victims are often left without explanations, Amnesty International said in a report released Tuesday.
The report from the London-based rights group follows up on its 2009 investigation alleging hundreds of police killings every year, many of which may have amounted to extrajudicial killings, it said.
"In the three years since that report appeared, unlawful police killings have continued virtually unchecked and with wholly ineffective investigations, according to Nigerian NGOs such as the Legal Defence and Assistance Project," Amnesty said in its report.
"Although some killings have been investigated and some police have been prosecuted, the vast majority are not the subject of adequate inquiry."
In a statement accompanying the report it said "hundreds of fatal police shootings... are not being investigated effectively because of a failure of the Nigerian justice system."
Nigerian police spokesman Frank Mba declined to comment when contacted by AFP, saying he had not yet seen the report.
The report, focusing on Rivers State in the southern oil-producing Niger Delta region of Africa's most populous nation, cites examples such as the fatal shooting of a 16-year-old boy who police suspected of stealing a bag.
According to Amnesty, an officer kicked the boy as he lay on the ground and they left him there even after the trader whose bag was stolen said the boy was not the culprit.
It said no officer is known to have been investigated over the death.
The report also describes conditions at a mortuary in Port Harcourt, the Rivers State capital.
The mortuary at Braithwaite Memorial Specialist Hospital, closed to new cases in July 2012, was overcrowded with bodies dumped in various places and lacking in equipment.
Bodies of those killed by police have also not been released to families within a reasonable time for burial, according to the report.
Amnesty said it feared that other mortuaries in the country operated in a similar fashion.
"To have one of your friends or family members killed by the authorities causes terrible anguish, but never to find out the truth of what actually happened to them causes a particular agony for relatives of the victims," Lucy Freeman, Amnesty's deputy director for Africa, said in the statement.
"Many of the victims killed by the police each year may have been unlawfully killed -- including in what constitutes extrajudicial executions."
Nigerian security forces have often faced accusations of brutality, including summary executions and arbitrary arrests.