Spanish electricity grid operator REE said on Tuesday its preliminary assessment had ruled out cyberattack as the cause of the nationwide power outage that hit most of Spain and Portugal on Monday.

Lights went off in most of the two countries at 12:33 p.m. (1033 GMT) on Monday in what was the worst blackout ever recorded in the Iberian Peninsula. Power supply only resumed in the evening.

REE’s System Operations Chief Eduardo Prieto told a news briefing the electricity system was hit by a dramatic power generation loss in southwestern Spain, that caused instability in the system that led to its disconnection from the French grid.

He said it was quite possible that the affected generation was solar, but it was to early to say for sure.

Prieto said the system was now stable and working normally.

(Reporting by David Latona, editing Inti Landauro and Andrei Khalip)

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