Raipur, Mar 12 (ANI): A preliminary investigation has been ordered into the Maoist attack on police in Chhattisgarh that killed 16 involved in a mine clearing operation on Tuesday. Maoist rebels ambushed police and killed 16 involved in a mine clearing operation near Tongpal in Sukma district, a remote part of Chhattisgarh. Two were left injured in the attack that comes a month ahead of the general elections.