52-year-old SI K N Rajan climbed into the cabin of the lorry and tried to snatch the key when the driver allegedly pushed him down and drove away the vehicle, they said.
The SI who suffered serious head injuries has been admitted to Pariyaram Medical College Hospital, District Superintendent of Police Unnnirajan said.
Acting on a tip-off that sand was being mined illegally, Rajan of Pariyaram police station had gone to the area in Parolikadavu along with a constable.
The lorry driver and cleaner are absconding, police said.