The Offa robbery gang leader's case became suspicious when he was not paraded alongside the other arrested suspects.
A coalition of Civil Society Organisations (CSOs) has threatened to sue the police if they do not produce Michael Adikwu, the leader of the arrested gang that carried out the Offa robbery incident.
The suspect, a former policeman, led the armed robbers that killed 33 people, including pregnant women and 12 police officers, on April 5, 2018, in Kwara state.
Adikwe had confessed to have killed several policemen as a revenge for his "unfair" dismissal from the force.
The gang leader's case became suspicious when he was not paraded alongside the other arrested suspects, and his name was also removed from the list of suspects in a statement from the police.
It was alleged that he had been shot dead in custody in order to coerce confessions from other suspects.
On Monday, June 11, the CSOs, in a petition signed by Collins Abu on behalf of the coalition read, threatened to drag the police to the International Criminal Court if it does not produce Adikwu.
"If he (Adikwu) has been killed, the police should come clean and name his executors, this is in keeping with the Freedom of Information Act (FOI)," the petition said.
"We demand an immediate investigation into the disappearance of this suspect, and we ask respected international human rights organisations to call on the Nigerian government and the police to produce him."
The coalition also accused the police of being compromised, thereby demanded an independent probe of the Offa robbery.
"Failure to produce him, we will petition the International Criminal Court and also petition all embassies with evidence so as to revoke visas of the IGP and other human rights violators," the group said.
The police spokesman, Jimoh Moshood, however, denied that the suspect was killed in custody.
"…We will parade him soon, once they recover all the weapons", Moshood told the Cable.
Some of the suspects have also alleged that they were working for the Senate President Bukola Saraki, who has been summoned by the police for questioning.
The Senate President has since dismissed the allegations as "baseless", saying the plot to link him with the armed robbery suspects was concocted by the Inspector General of Police, Ibrahim Idris to embarrass him.