I heard they were not using 4 wheelers, but were in fact using Jetpacks and wearing Big Bird costumes as disguises.
The only witness they had was a woman from Clements that phoned in saying she saw Big Bird fly by her house, and she was threw 10ozers at him but the cans were empty and didn't have enough carry to reach the bird. At first the cops thought she was just drunk and phoning in another alien sighting (I guess she has a history of that) but later they realized it was true because CSI:Leonardtown confirmed that yellow fuzz found in the bank was actually a feather used in the manufacture of Big Bird costumes in Tiawan, and the burn marks in the grass were the exact dimentions of a Jetpacks blast.
I heard this from a game warden's sisters husband who said not to tell so don't tell anyone I said it.