The massive power outage that swept across Spain and the larger Iberian Peninsula in late April was caused by multiple technical and planning failures, according to a report released on Tuesday.

Spanish energy minister Sara Aagesen told reporters that small grid failures caused a chain reaction, leading to the blackout. The incident was not caused by a cyberattack, Aagesen added.

“All of this happened in 12 seconds, with most of the power loss happening in just five seconds,” Aagesen said.

The Spanish blackout occurred on April 28 shortly after 12:30 p.m. and lasted through the night. Madrid and Lisbon, the respective capitals of Spain and Portugal, were hit by the power outage. A small portion of southwestern France was additionally affected.

Among those affected were tens of thousands of travelers. About 500 flights across Spain and Portugal were canceled, affecting approximately 80,000 passengers. Transit schedules were also delayed, causing passengers to sleep in train stations overnight.

Electricity was fully restored the day after, but the episode prompted concerns of a possible cyberattack. At the time, Portuguese Prime Minister Luis Montenegro said there was “no indication” that an attack was the cause.

The government investigation found no evidence of a cyberattack.

According to Tuesday’s report, the cause was a miscalculation. Spanish power grid operator Red Electrica de Espana did not have enough thermal power plants left on during the peak hours of April 28. As a result, a surge in voltage led to the mass blackout.

“The system did not have sufficient dynamic voltage control capacity,” Aagesen said at a news conference in Madrid.

The state-owned power grid operator “told us that they made their calculations and estimated that [switching on more thermal plants] was not necessary at this time,” she added. “They only set it for the early hours of the day, not the central hours.”

EDITORIAL: SPAIN PROVES RENEWABLES ARE NOT RELIABLE

Spain relies on renewable energy sources, such as wind and solar energy, for more than half of its electricity. Last year, the European country generated 56% of its electricity from “green” energy.

Spanish Prime Minister Pedro Sanchez defended renewable energy, vowing to continue with his ambitious energy goal. By 2030, Spain is expected to generate 81% of its electricity from renewable energy sources.