A power surge that caused a paralysing blackout across all of Spain and Portugal in April was an unprecedented event in Europe and possibly the world, but the root cause remains unknown, an expert panel said Friday.

“This was the most severe blackout incident in Europe in the last 20 years,” Damian Cortinas, president of the association of electricity grid operators ENTSO-E, said during a presentation of the panel’s preliminary report into the incident.

He said “cascading overvoltages” were behind the blackout, which he called a “first of its kind” on the continent.

Cortinas added that “we think (it is unprecedented) in the world as well”, but cautioned that the experts did not have information from every country.

The April 28 outage cut internet and telephone connections, halted train

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