NORTH CAROLINA — When Moore County, N.C., suddenly went dark last Saturday night, Mayor Carol Haney was perplexed. There'd been no storm, no warning, just darkness on what had been a festive holiday season evening. It turned out a shooting attack on two electrical substations knocked tens of thousands of people off the grid for the better part of a week.

"This beautiful part of the world gets sabotaged," said Haney. "So, it could happen to anyone. And that's probably the most frightening for everyone– that this could happen to anyone."

It could. Across the United States some 55,000 electrical substations are humming right now, mostly transforming high voltage from big power lines, into lower voltages for homes and businesses. Many of them are sitting ducks for saboteurs.

"The electric grid is the Achilles heel of the United States," says Mike Mabee, a self-described "grid-security gadfly" who pours over electric company data to highlight vulnerabilities.

Like many he's worried about Russian or Chinese cyberattacks, but Mabee says the easiest way to hurt Americans is something a lot less exotic, shooting up substations with widely available assault rifles.

"If a terrorist organization, whether homegrown or a foreign terrorist organization wants to visit damage on the United States electric grid, the easiest way to do it is by a physical attack," says Mabee.

None of this is lost on domestic extremist groups or the government. The Department of Homeland Security issued a law enforcement bulletin in January warning that domestic terrorists had developed "credible, specific plans to attack electricity infrastructure", and viewed the power grid as a "particularly attractive target."

Substations are soft targets, because the main components in them, huge voltage transformers, cool themselves with circulating oil. High-powered rifle rounds can easily pierce transformers, spring leaks, make them overheat and shut down. The bigger transformers are about the size of railroad boxcars. Carnegie Mellon University professor M.Granger Morgan says they aren't easy to replace.

"We don't make many of them in this country, and there are long backlogs in getting new ones," says Morgan.

The backlogs can stretch to 18 months, with price tags that can run into the millions of dollars. And the cost to replace equipment pales compared to the potential toll of taking down the grid.

In 2013 shooters attacked a substation just outside San Jose California. Early media reports called it vandalism, but Jon Wellinghoff, Chairman of the Federal Energy Regulatory Commission at the time, immediately recognized it as something more sinister.

"So, I took a team out there the very next day to investigate," says Wellinghoff. "We actually found firing positions they had marked on the ground. Two shooters, maybe more. We don't know how many people actually were in the team that executed this action. But it was extremely sophisticated."

The saboteurs cut fiber optic lines to the substation pumped more than 100 rounds through a chain link fence protecting the substation. They hit vulnerable parts of the transformers, and fled seconds before police arrived. The bullet holes drained more than 50,000 gallons of cooling oil and knocked out 17 of 21 transformers. Wellinghoff says the attackers came close to taking Silicon Valley off the grid, in an outage that he says could have lasted several weeks.

It was a stunning attack. Wellinghoff thought it would force a reckoning on the way the government regulates grid security. Currently, no one agency has that authority because the duties are split between federal and state regulators.

"We need somebody in charge, and it's up to Congress to put somebody in charge. We went to Congress and asked for additional authority, but didn't get it," says Wellinghoff. "The industry writes the standards, and then they're submitted for approval. And that's what happened here."

So, 3,000 or so power companies and cooperatives across the United States decided themselves which substations needed protection and what added security was warranted. They built concrete walls around some substations to stop bullets but Wellinghoff says security upgrades didn't reach many of them.

"And looking at them, most of them don't seem to be very well protected," Wellinghoff says. "Many of them still have chain link fences, like the one in North Carolina."

Those individual vulnerabilities add up to one massive problem. Wellinghoff says a series of precisely targeted substation attacks could trigger a cascade of failures, taking down most of the US power grid.

Power companies say they're on top of the situation. A spokesman for Duke Energy, the company which owns the substations attacked in North Carolina, says the company works constantly to improve security, and to meet new threats. He says the whole industry will learn from the North Carolina attacks.

The FBI is investigating alongside state and local authorities. They've collected dozens of spent shell casings at the site. State police are reportedly seeking search warrants, while the FBI is asking for cell phone records.

The attacks keep coming. Wednesday shooters targeted another Duke substation, this one in South Carolina, without triggering a blackout.

Meantime some of the people who know the most about the U-S power grid are taking concrete steps to be able to do without it. Both Jon Wellinghoff, and Mike Mabee have had solar panels, batteries, and generators installed in their homes.

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