The incident follows vandalization of electrical substations last month that left thousands of residents without power in North Carolina, South Carolina and Washington state.

North Carolina utility EnergyUnited said no customers lost power due to the incident at its Pleasant Hill Substation on Tuesday.

Crews were dispatched to assess the situation at the substation after an alarm that notified personnel of an equipment issue. They discovered damage to the substation transformer from an apparent gunshot, the company said in a statement, adding the damage was quickly contained.

Officials said the attack occurred around 3 a.m. ET (0800 GMT) on Tuesday. The FBI and state authorities were probing the incident.

The latest incident comes after four electrical substations were vandalized around Christmas, leaving over 14,000 customers without power near Tacoma, Washington. Two men were arrested in connection with those attacks, authorities said earlier this month.

Also in December, a utility in North Carolina reported outages from what local authorities said were orchestrated shootings investigated by federal law enforcement. Duke Energy Corp, which provided power to the area, said at the time a total of 45,000 people had lost power after the shootings.

The FBI also investigated shots fired near a power facility in South Carolina days later.

North Carolina Governor Roy Cooper said state authorities and the company were offering monetary rewards of up to $75,000 for information that may lead to the arrest and conviction of those responsible for the December attacks on the substations.

Cooper had said "a serious national conversation" about protecting critical infrastructure was needed after the attacks.

(Reporting by Kanishka Singh in Washington; Editing by Marguerita Choy)

By Kanishka Singh