PG&E Corp. failed to adequately inspect and maintain its transmission lines for years before a faulty line started the deadliest fire in California history, a state investigation has found.

In a 700-page report detailing the problems that led the Caribou-Palermo transmission line to malfunction on Nov. 8, 2018, sparking the Camp Fire, investigators with the California Public Utilities Commission said they found systemic problems with how the company oversaw the safety of its oldest lines.

State...