Spanish grid operator Redeia blamed power plants for the massive blackout that affected the Iberian peninsula in April, as it disputed a government report that said its failure to calculate the correct energy mix was a key factor.

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.

Spain’s most-detailed account yet of the missteps leading to April’s nationwide blackout failed to fully explain why the power grid was so unstable in the first place.