MADRID, Spain: A routine oversight by Spain's power grid operator, REE, has been identified as the trigger behind the large-scale blackout that swept across the Iberian peninsula in late April, according to findings from a government-led investigation released this week.

The report found that REE underestimated the amount of electricity needed to ensure voltage stability during peak hours on April 28 — a miscalculation that set off a chain reaction, plunging cities into gridlock and stranding thousands in elevators and trains.

"The system did not have sufficient dynamic voltage control capacity," Spain's Energy Minister Sara Aagesen told reporters in Madrid.

She said REE failed to keep enough thermal power plants online during critical hours. "They only set it for the early hours of the day, not the central hours."

The report, published in full on June 17, also found that some power plants legally required to help stabilize the grid failed to absorb the reactive power needed to manage the surge.

"Many of them were economically remunerated to do so," Aagesen said. "They did not absorb all the reactive power that was expected in a context of high voltages."

Maintaining grid stability requires the system frequency — usually 50 Hertz (Hz) across Europe — to remain within narrow bounds. Deviations can damage infrastructure and equipment.

The minister confirmed there was no evidence of a cyberattack.