A 3-inch hook purchased for 56 cents around the end of World War I could help determine whether PG&E Corp. faces criminal charges for starting the deadliest wildfire in California history.

Known as a “C-hook,” the badly worn piece of metal broke on Nov. 8, 2018, dropping a high-voltage electric line that sparked the Camp Fire, destroying the town of Paradise and killing 85 people.

PG&E...