At least five prisoners died of electrocution at the Ikoyi prison in Lagos on Monday.
PREMIUM TIMES reported the electrocution from which other prisoners also sustained injuries.
Although the exact cause of the electrocution has not been determined, an official has provided information on the level of overcrowding in the prison.
The Controller General of the Nigerian Correctional Service (NCoS), Jaafaru Ahmed, said as at the time of the incident, the Ikoyi Medium Security Custodial Centre built in 1955 for 800 inmates, had a population of 3,113 with 2,680 of them awaiting trial.
The NCoS on Monday ordered an immediate investigation into the deaths at the prison.
Mr Ahmed gave the directive in a statement signed and released to journalists by the national spokesman of NCoS, Francis Enobore, on Monday in Lagos.
He said that the directive became necessary after the Minister of Interior, Rauf Aregbesola, and the CG had visited the scene of the incident.
The statement said, "Early this morning, CG Ja'afaru Ahmed received with the deepest grief, the unfortunate death of five inmates of Ikoyi Medium Security Custodial Centre, Lagos as a result of electrical fault in one of the cells.
"It was indeed tragic and most ill-fated despite the fact that it was the first time in the history of the NCoS to experience such a disaster.
"The CG, therefore, has ordered an immediate investigation into the sad incident with a view to taking necessary actions in order to forestall future occurrences."
The spokesman added that the CG expressed his condolences to the families of the deceased inmates.
He also prayed for the quick recovery of the seven others that were receiving treatment in the hospital following various injuries they sustained from the incident.
"We also assure the members of the public that inmates' safety and humane custody will continue to be a priority of this administration.
"We further restate our commitment to pursue the policy thrust of the federal government at improving the general welfare of inmates," Mr Ahmed said.