"Investigations are still underway, but we have established that police officers were involved and have been implicated in the explosion," spokesman Moses Dlamini said.

"I can't go into the small details because the investigations continue, but the officers were involved in the smuggling of the explosive device to the prisoners."

Dlamini said arrests were imminent.

On October 22, an explosive device detonated in a prison truck taking 36 awaiting-trial prisoners from the Randburg Magistrate's Court to Johannesburg Central Prison.

Two prisoners tried to escape, but were recaptured by the police. One of them was shot and wounded by the police.

On October 26, Dan Mabona, 32, Ephraim Moyo, James Motsiti, 33, and Raymond Moyo, 28 appeared in the Johannesburg Magistrate's Court.

The men face charges of murder, attempted murder, possession of explosives, and malicious damage to property. The matter was postponed to December for further investigation.

The men had been in custody prior to the incident and all faced charges of robbery.