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Shooters armed with assault rifles and some knowledge of electrical utilities have prompted new worries about the vulnerability of California’s vast power grid.

An April 2013 attack on an electric substation near San Jose that nearly knocked out Silicon Valley’s power supply was initially downplayed as vandalism by Pacific Gas & Electric (PG&E), the facility’s owner. Gunfire from semi-automatic weapons did extensive damage to 17 transformers that sent grid operators scrambling to avoid a blackout.

Last week, a former top power regulator offered a more ominous interpretation: The attack was terrorism, he said, and if circumstances had been just a little different, it could have been disastrous.

Jon Wellinghoff, who was chairman of the Federal Energy Regulatory Commission (FERC) when the shooting took place, said the attack was clearly executed by well-trained individuals seeking to do significant damage to the area, and he fears it was a test run for a larger assault.

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“It would not be that hard to bring down the entire region west of the Rockies if you, in fact, had a coordinated attack like this against a number of substations,” Wellinghoff said Thursday. “This (shooting) event shows there are people out there capable of such an attack.”

Wellinghoff’s warning about the incident at PG&E’s Metcalf substation was reported by The Wall Street Journal, expanding on a December report by Foreign Policy magazine.

Several senators on Friday called on regulators to review security operations at electrical utilities and consider imposing new rules to protect against more attacks.

“Last year’s sophisticated attack on the Metcalf substation in California’s Silicon Valley was a wake-up call to the risk of physical attacks on the grid. The incident came uncomfortably close to causing a shutdown of a critical substation which could have resulted in a massive blackout in California and elsewhere in the West,” said a letter signed by Senate Majority Leader Harry Reid of Nevada and fellow Democratic Sens. Dianne Feinstein of California, Ron Wyden of Oregon and Al Franken of Minnesota. The letter was sent to executives at FERC and North American Electric Reliability, an industry group that sets rules for how power companies operate.

FBI officials said they are taking the shooting seriously.

“Based on the information we have right now, we don’t believe it’s related to terrorism,” said Peter Lee, an FBI spokesman in San Francisco. But, he added: “Until we understand the motives, we won’t be 100 percent sure it’s not terrorism.”

Months after the shooting, the bureau has named no suspects.

As law-enforcement agencies try to piece together who fired at the electricity facility, lawmakers and analysts express bewilderment that little is being done to protect against a repeat performance.

“We’ve got a vulnerability, and we’ve got to get serious about fixing it,” said Granger Morgan, who heads the Department of Engineering and Public Policy at Carnegie Mellon University. “Almost everything we do in modern society relies on electricity.”

The attack on the PG&E facility targeted the sophisticated transformers that are at the backbone of the nation’s electricity grid. The giant pieces of equipment are essential, costly and could take months to replace. Knock out enough of them, experts warn, and a region can be crippled for an extended period. They are also typically out in the open like sitting ducks.

On that April night, the attackers disabled 17 of the transformers by shooting through a chain-link fence. The bullet holes caused the transformers to leak thousands of gallons of oil, and ultimately overheat. Grid operators scrambled to reroute power from elsewhere to keep the system from collapse. The power stayed on, barely, because it happened when demand for electricity was very low.

“Fortunately it was spring and we did not have air conditioners running full throttle,” said Stephanie McCorkle, a spokeswoman for the California Independent System Operator in Folsom, which runs most of the state’s electrical grid. “That’s why the situation was manageable.”

Wellinghoff, now a partner at the San Francisco law office Stoel Rives, said the grid’s interdependence on substations across large swaths of the country and a scarcity of spare equipment make it possible to trigger an enduring blackout across several states simply by destroying key transformers in one of them.

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