Collins said a popping sound from the heater "sounded just like an AR-15 (semiautomatic rifle) going off", the newspaper reported.
According to Commissioner George Brown, a piece of equipment at the school campus, a water heater, has been malfunctioning in recent days, emitting a sound similar to gunfire.
The sheriff's spokesman said law enforcement officers were still searching the campus as a precaution but preliminary reports indicate that it was faulty equipment.
Pender County schools spokeswoman Miranda Ferguson said in an email that Topsail Elementary and Middle are opening for staff and staff at Topsail High School will be informed soon about opening.
Pender County Sheriff's Office said officers responded to "reports of an active shooter" at Topsail High School in the town of Hampstead at around 7am.
Collins told Starnewsonline.com that popping noises heard at Topsail High School early Friday were from a "malfunctioning water heater".
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Parents are being told to pick up their students at the Lowes Foods and to bring proper identification.
No injuries were reported and there is no evidence of an active shooter.
All schools in the area were put on lock down and emergency protocols put in place.
Topsail High School is located near the coast in Hampstead, about 20 miles north of Wilmington.
School buses headed for the school were being diverted to a nearby Lowe's grocery parking lot.