The FBI is investigating after two power plants in North Carolina were damaged by gunfire, leaving tens of thousands without power.
Since the attack on Saturday evening, no motive or suspect has been identified, but police have stated that it was deliberate.
According to officials, 35,000 people in Moore County are without power, and the damage could take several days to repair.
On Tuesday, a state of emergency has been declared, and schools will be closed for the second day in a row.
The declaration of emergency includes a countywide overnight curfew that will expire on Friday afternoon.
“This kind of attack raises a whole new level of threat,” North Carolina Governor Roy Cooper said in a briefing on Monday, adding that protecting critical infrastructure must be a “top priority”.
The two substations are located around five miles apart (8km), and in one case a gate was breached to access the facility.
“The person, or persons, who did this knew exactly what they were doing,” Sheriff Ronnie Fields said at a news conference on Sunday. “We don’t have a clue why [they targeted] Moore County.”
He said the FBI was working with local authorities to determine who was responsible, adding that someone pulled up and “opened fire on the substation, the same thing with the other one”.
“It wasn’t random,” he said.
It could take until Thursday to restore power, officials say, as the damage to the two substations is significant. Around 45,000 people were initially affected by the outage.
“We are looking at a pretty sophisticated repair with some fairly large equipment and so we do want citizens of the town to be prepared,” Jeff Brooks, a spokesman for the local energy company Duke Energy, said at the news conference.
A fire chief, Mike Cameron, said there were several road accidents when the power went out including a four-vehicle pile up. “The car wreck was totally because the stop lights were out,” he told the Charlotte Observer newspaper.
A major hospital switched to using generator power, while water and sewage services are also running on back-up generators.
Federal officials are concerned about copy cat attacks. In Washington, White House National Security Council spokesman John Kirby said they were carefully watching developments in the investigation.
Temperatures hit a low of 32F (0C) on Monday morning, and the county opened an emergency shelter at a sports complex for those in need.
Grocery stores in the region were open, and running on generators on Monday, according to the local newspaper The Pilot.
Some local restaurants were giving away perishable goods as their freezers stopped working, the paper reported.
It is unclear how long schools in the county will remain closed, with officials saying decisions on openings will be made on a day-by-day basis.
While a suspect has not been identified and the motive in unclear, Sheriff Fields addressed social media rumours that the vandalism was an attempt to stop a drag show from taking place.
“[Investigators] have not been able to tie anything back to the drag show,” he said. The event had been scheduled for 19:00 on Saturday, which is around the same time the power went out.