Authorities in Moore County have filed search warrants in connection with the attack on two Duke Energy electrical substations Saturday night that left about 45,000 customers without power for several days.
No suspects have been identified.
The FBI and Moore County filed search warrants to get cellphone tower data, the FBI told Channel 9. Search warrant applications are a normal step in a law enforcement investigation.
Gun shop owner sleeps in store to ward off thieves during Moore County blackouts
Power grid attack: Duke Energy previously fined for security violations, report alleges
NC families seek shelter from cold amid power outages caused by substation attack
FBI investigating power grid attack in Moore County, sheriff says
UNC journalism student from Concord covers Moore County blackouts for NYT
One or more people broke into the substations through the gates and opened fire on them, cutting off power at about 7 p.m. Saturday, authorities said.
A motive has not been released in the attacks, but Moore County Sheriff Ronnie Fields said that whoever was responsible "knew exactly what they were doing to ... cause the outage that they did."
The equipment was repaired and power was restored Wednesday. The state of emergency for the county ended at 5 a.m. Thursday.
Earlier in the week, Gov. Roy Cooper announced a reward of up to $75,000 for information leading to the arrest and conviction of whoever is responsible for the attacks.
On Thursday, Duke Energy said it found no sign of property damage at a hydropower station in South Carolina where gunfire was reported nearby. Multiple federal agencies are monitoring the development.
At least four electrical substations have been targeted in separate attacks in Oregon and Washington since late November, Oregon Public Broadcasting and KUOW Public Radio reported Thursday. Attackers used firearms in at least some of the incidents, and some power customers in Oregon temporarily lost service.
VIDEO: Power restored: Moore County residents try to return to normal after substation attack
The Associated Press contributed to this report.