Too late

The plane was late and dectives were waiting at the airport all morning. They were expecting a valuable parcel of diamonds from south Africa. A few hours earlier, someone had told the police that thieves would try to steal the diamonds. When the plan arrivaed, some of the dectives were waiting inside the main building while the others were waiting on the airfield. Tow men took the parcle  off the plane and took it into the customes house. While two detectives were keeping guard at the door, two others opend the parcel. To their surprise, the valuable parcel was full of stones and sand.