A North Carolina power substation was damaged by gunfire early Tuesday in the third known power substation shooting in the state since early December.

Crews responded to an alarm at the substation in the city of Thomasville, southwest of Greensboro, where they discovered the substation transformer was struck by an "apparent gunshot," the electricity provider EnergyUnited said in a statement.

The utility said no customers experienced a power outage as a result.

The Randolph County Sheriff's Office said law enforcement investigators canvassed the scene and gathered evidence and believe the shooting happened at about 3 a.m.

An investigation is ongoing.

The FBI and North Carolina State Bureau of Investigations were notified and the FBI's Joint Terrorism Task Force is conducting its own investigation, the sheriff's office said.

In early December, two power substations in neighboring Moore County were damaged by gunfire on the same night, knocking out power for tens of thousands of residents for multiple days.

No arrests have been announced related to that shooting. The FBI is still seeking information.

North Carolina Gov. Roy Cooper said at the time that the Moore County attack "raises a new level of threat," and said state and federal officials would work to "harden our infrastructure where that's necessary and work to prevent future damage."

Energy security experts say it's tough to stop attacks intended to knock out electricity, especially in rural areas.

"The grid is extremely large," said Errol Southers, professor of national and homeland security at the University of Southern California, speaking to NPR last month. "It has about 6,400 power plants across the country, some 55,000 substations and over 450,000 miles of high-voltage transmission lines serviced by 3,000 companies. ... It's extremely challenging to monitor and protect. And many of these places are very remote, and so officers have to get there. And by the time they do, the attackers are already gone."

Four power substations in Washington state were damaged by attacks on Christmas Day. Between mid-November and Dec. 8, at least six other attacks occurred on substations in Oregon and Washington.

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