A government video shows the potential destruction caused by hackers seizing control of a crucial part of the U.S. electrical grid: an industrial turbine spinning wildly out of control until it becomes a smoking hulk and power shuts down.

The electrical attack never actually happened. The recorded demonstration, called the “Aurora Generator Test,” was conducted in March by government researchers investigating a dangerous vulnerability in computers at U.S. utility companies known as supervisory control and data acquisition systems. The programming flaw was quietly fixed, and equipment-makers urged utilities to take protective measures.

There was no evidence any U.S. utility company suffered damage from hackers or terrorists using this technique, U.S. officials said. But these officials cautioned that affected systems are not routinely monitored as closely as many modern corporate computer networks, so there would be little forensic evidence to study after such a break-in.