Investigators from the North American Electric Reliability Council have now traced the events that led to the blackout that left a majority of New York and Ohio in the dark to the failure of several high-voltage transmission lines near Cleveland, but it remains unclear how the problem spread from the Midwest to the East coast and parts of eastern Canada. The first breakdown occurred at 3:06 p.m. EST, when a 345-KV-transmission line west of Cleveland shut down for reasons still unknown. Not too long after the first line failed, a second 345-KV line in the same area, probably one helping to carry the load from the first failed line, sagged into a tree, causing the line to shut down. Over the next hour, systems throughout the throughout the eastern United States and Canada began to see huge swings in voltage and direction of power flow, more lines went down, and power plants shut themselves off. Shortly before 4:11 p.m. EST, the blackouts began. The council’s investigation is ongoing.

“Instability in the voltage is something you really have to worry about,” says Karl Stahlkopf, senior vice president of Hawaiian Electric and a former vice president of the Electric Research Institute in Palo Alto, Calif. “In the ’96 outages in California, we had very large voltage swings, which caused relays to trip, which caused plants to go offline, which caused blackouts.”