SAN JOSE, Calif.—The attack began just before 1 a.m. on April 16 last year, when someone slipped into an underground vault not far from a busy freeway and cut telephone cables.

Within half an hour, snipers opened fire on a nearby electrical substation. Shooting for 19 minutes, they surgically knocked out 17 giant transformers that funnel power to Silicon Valley. A minute before a police car arrived, the shooters disappeared into the night.

To avoid a blackout, electric-grid officials rerouted power around the site and asked power plants in Silicon Valley to produce more electricity. But it took utility workers 27 days to make repairs and bring the substation back to life.

Nobody has been arrested or charged in the attack at PG&E Corp. 's Metcalf transmission substation. It is an incident of which few Americans are aware. But one former federal regulator is calling it a terrorist act that, if it were widely replicated across the country, could take down the U.S. electric grid and black out much of the country.