The Arizona Corporation Commission wants the state's utility companies to explain whether they're prepared to defend the electric grid against terrorist attacks.

The Arizona Republic reported the concern is prompted by an attack last April on an electrical substation in California.

Gunmen opened fire on the facility near San Jose and knocked out 17 transformers. Pacific Gas and Electric prevented a blackout by rerouting power.

Commission Chairman Bob Stump and fellow commissioner Robert Burns sent a letter last week to Arizona Public Service Co., Tucson Electric Power and UniSource Energy Services, Salt River Project and the state's electric cooperatives.

The letter asked the companies questions ranging from whether the utilities' cybersecurity and physical security plans overlap to whether their critical service providers are included in their security plans.