A government investigation found that efforts by Red Eléctrica de España, Spain's grid operator, to address a small instability in the system actually led to more severe issues, causing a self-reinforcing cascade where high voltages caused power plants to drop off the grid.

Spain’s government said Tuesday that the massive April power outage across Spain and Portugal that left tens of millions of people disconnected in seconds was caused by technical and planning errors that left the grid unable to handle a surge in voltage.