Chief Prosecutor Bruno Louapambe Mahouli said in a statement today that the ambush took place early today while the detainees were being transferred from N'djamena, the capital, to Koro Toro.
He says at least one security officer and three detainees were killed in the attack.
There was no immediate claim of responsibility, although there is widespread instability outside the Chadian capital.
The Justice Ministry vowed to find the perpetrators and to hold them accountable. Officials did not provide information about the charges the detainees were facing.