Last April a skilled and well-prepared assault team of unknown identity and motive systematically shot up a Pacific Gas and Electric power substation near San Jose, Calif., putting it out of business for months.

The attack has rightly raised concerns in Congress, the executive branch and the electric power industry for the physical safety of the nation's electric power grid. Analysts say attacks on a small number of well-chosen power substations could cut electric power to large parts of the nation for an extended period.

Equally alarming is the evident presence in the continental United States, at least briefly, of at least one armed team of hostile activists, whether domestic or foreign terrorists - or even agents of a foreign power, as unlikely as that seems.

One electric industry official industry told The Wall Street Journal the attack "appears to be preparation for an act of war."

The assailants cut communications links to the isolated power substation, then spent 19 minutes attacking the cooling systems of 17 of its 19 transformers with assault rifles. By aiming at the cooling systems and not the transformers themselves, the shooters avoided creating explosions big enough to alert authorities. They fired about 120 rounds and scored 110 hits, causing the 17 transformers to slowly overheat and fail. Then the team vanished.

PG&E was able to avert a Silicon Valley blackout by shifting power loads to other substations. The transformers, a critical link in the electric power grid, are costly and take months to replace.

PG&E initially reported the incident as vandalism. But as the Journal reported, an independent investigation by the head of the Federal Energy Regulatory Commission in conjunction with Defense Department experts made it clear that the attack was planned and executed by individuals with training in special military operations.

Jon Wellinghoff, chairman of the FERC from 2009-13, said those defense specialists described it as a professional-looking mission.

Physical attacks on the electric power grid are not uncommon in parts of the world where terrorists regularly operate, and even here there are occasional minor individual incidents. But the San Jose attack represents the first attack on this scale in U.S. history.

The nation has already experienced cyber probes of its electrical grid and is taking steps to counter them.

Now the physical security of the system has been challenged. The electric power industry and the government must rapidly come together on security standards for all elements of the grid.

Meanwhile, the FBI, which has yet to decide that the San Jose attack was a terrorist action and is "continuing to sift through the evidence," according to a spokesman, needs to get busy and find the perpetrators.

As for official refusal to brand this a "terrorist" attack, that's of scant consolation considering the culprits haven't been identified.

And across the wide range of potential terror targets in the U.S., our electricity grid is a powerful target.