A retired federal regulator has gone public with concerns that a gun attack on a Northern California electrical substation may have been a dress rehearsal for a larger assault on the U.S. power grid aimed at blacking out much or all of America.

Jon Wellinghoff, ex-chairman of the Federal Energy Regulatory Commission, told The Wall Street Journal that such an attack would likely only need to target a few substations to cause blackouts in most or all of the country.

Wellinghoff described the April 16 attack on the Pacific Gas & Electric Corporation's transmission substation at Metcalf, Calif. as "the most significant incident of domestic terrorism involving the grid that has ever occurred."

According to a chronology assembled by the Journal based on state and federal filings by PG&E, the operation lasted 52 minutes. It began when communications cables in two vaults near the substation were cut. Then, unknown snipers fired over 100 shots from rifles aimed at the substation's transformers, knocking out 17 of them. Utility workers needed 27 days to repair the damage, and officials only avoided a blackout in the area by rerouting power around the site and asking power plants in nearby Silicon Valley to produce more electricity.

The FBI has not made any arrests or named any suspects publicly. An spokesman for the FBI's San Francisco office says he doesn't think the Metcalf incident was the work of a terrorist group.

Others have disagreed with Wellinghoff's assessment of the threat to the grid. Gerry Cauley, chief executive of the North America Electric Reliability Corp., a standards-setting group that reports to FERC, told the Journal that the vast majority of people would see their power quickly restored if several substations went down, though he added that he didn't "want to downplay the scenario [Wellinghoff] describes. I'll agree it's possible from a technical assessment."

Others have backed up the former FERC chair, including Rich Lordan, senior technical executive for the Electric Power Research Institute, who told the Journal "The breadth and depth of the attack was unprecedented" in the U.S. and said he believed the motivation was "preparation for an act of war."

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