Teachers thought they heard gunfire near a New Jersey elementary school, but it turns out it was just a backfiring recreational vehicle.

Deptford, N.J., police spokesman George Johnson says teachers at the Shady Lane School were outside when they thought they had heard gunshots. The school, which has about 500 students from pre-kindergarten to sixth grade, was locked down. Police arrived on the scene.

Johnson says the RV owner, who was nearby, saw the large number of police officers and realized vehicle may have triggered fear. He told officers what had happened.

Deptford is across the Delaware River from Philadelphia.