Drug or drag (depending on tense) and dredge were all used in the shop, when joking about what lay at the bottom of the river and the best way of hiding evidence. People often use improper linguistics in conversation. e.g. They drug his behind out of the bar, over the proper they dragged his behind.
Some were I suppose imagining a process like dragging a river for a body with hooks . Technically a barge load would certainly be dredging, dragging the river just gives it all a more sinister mood.
The whole Caterpillar operation just seemed so absurd, I found it hard to imagine a group of executives agreeing that this would be a sensible thing to do, not thinking that this action could possibly cause Caterpillar to be dragged into court.