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One of the most significant acts of domestic terrorism involving the power grid took place last Spring in Northern California and you probably didn’t even know about it. I surely didn’t.

According to a report in the Wall Street Journal, last April, in the middle of the night, snipers opened fire for almost 20 minutes on a substation next to a freeway south of San Jose, California, and knocked out seventeen transformers that direct power around Silicon Valley. They also cut the nearby telephone cables. It took a month to fix all the damage — the attackers are still unknown.

The former chairman of the Federal Energy Regulatory Commission, Jon Wellinghoff — who now works for the law firm Stoel Rives — has been trying to bring attention to the incident because he thinks it could have been a dress rehearsal for a larger coordinated attack on the U.S. power grid. Shortly after the event occurred, Wellinghoff — who was still FERC Chairman at the time — took a group from the U.S. Navy’s Dahlgren Surface Warfare Center in Virginia (which trains Navy SEALs) to investigate the scene and they determined it was a professional job.

Wellinghoff thinks that a larger coordinated attack could be so detrimental that it could lead to widespread blackouts across the U.S. Not everyone agrees with him, though, and others quoted in the Wall Street Journal article think the grid is more resilient.

Still, much of the discussion around grid security in recent years has been focused on cyber attacks. The idea is that as more and more digital technology, wireless networks, and software are added to the grid, the more the grid is vulnerable to the type of hacking that plagues the Internet. But perhaps the real grid vulnerabilities still lie in the actual physical systems.