Power had almost fully returned to Spain early Tuesday morning as many questions remained about what caused one of Europe’s most severe blackouts that grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across Spain and Portugal.

More than 99 percent of power networks in Spain were restored by Tuesday morning and most had “stabilised” in Portugal, according to the respective power operators, following Monday’s outage that stranded people in buildings, elevators and public transport and cut access to phones and the internet.