Nearly all of Puerto Rico was without power Tuesday after a widespread failure in the fragile electricity grid triggered an island-wide blackout.

The outage began at 5:30 a.m., leaving about 1.3 million of Puerto Rico’s 1.4 million customers in the dark, according to Luma Energy, the private company that operates the system. Crews have begun restoring electricity in some areas, but it may take 24 to 48 hours to fully resume service, officials said.

“It was a major event,” Josue Colon, director of the Puerto Rico Electric Power Authority, said in an interview with WKAQ radio.

Initial indications point to a fault in an underground cable, though the exact cause remains under investigation, Luma said.

The company said it’s activated its Emergency Operations Center and is coordinating with power providers to restart the system in phases. Luma plans to provide updates every two hours via social media.

The U.S. territory has one of the most expensive and least reliable power systems in the nation, with frequent outages plaguing residents. Tuesday’s outage was the most extensive since Hurricane Ernesto hit in August, leaving roughly half the island without power.

Efforts to modernize the power system have been slow, stoking anger among residents, whose daily lives are often disrupted.

Julio Romero, 59, who was delivering newspapers on the streets of San Juan on Tuesday, said he was worried about what the blackout could mean for New Year’s celebrations.

“If this continues on until tonight, then god save us, this will be total chaos,” he said. “What a way to end the year.”

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