The FBI has been called in after two power stations in North Carolina were shot at, leaving tens of thousands of people without electricity.
Across Moore County, businesses, shops and restaurants have been forced to close, traffic lights were out and emergency shelters were set up following the attack.
A state of emergency has been declared and schools will be closed for a second day in a row on Tuesday.
The declaration includes a county-wide overnight curfew until at least Friday.
A total of 38,000 households remain without power amid freezing nighttime temperatures. At the peak, 45,000 were powerless.
North Carolina governor Roy Cooper warned of a “new level of threat” posed by the incident and called for better protection of critical infrastructure, including the power grid.
“Protecting critical infrastructure like our power system must be a top priority,” he said. “These kinds of things cannot happen.”
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Asked if the attack was being investigated as an act of domestic terrorism, Mr Cooper said “investigators are leaving no stone unturned”.
Police said the shooting was committed by someone with “criminal intent”.
Their comment came after online speculation bubbled that the attack was a protest against a planned drag act show set to go ahead on Saturday.
Organisers of the show at the Sunrise Theatre in Southern Pines said far-right activists had been trying to shut down the show for weeks, but police maintained they had found no evidence to support the theory.
Federal agencies including the FBI and the US Department of Energy are investigating the shootings, along with local and state law enforcement.
So far, none have suggested a motive and no arrests have been made.
US President Joe Biden was briefed on the incident, which occurred on Saturday, the White House said.
Moore County is about 60 miles (95km) southwest of the state capital of Raleigh.
Sheriff Ronnie Fields said the perpetrators “knew exactly what they were doing to … cause the outage that they did”.
