San Juan, Puerto Rico - A major power outage plunged much of Puerto Rico into darkness Tuesday, with the island's electric utility saying restoration could take up to two days.

The "island-wide blackout" began at 5:30 AM, Luma Energy said in a social media statement.

It added that preliminary findings pointed to a fault on an underground cable as the cause of the outage – although the situation remains under investigation.

"We have already begun the restoration process for some customers, and the entire process will take 24 to 48 hours," Luma said.

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In a separate notice, Luma noted that the incident left its 1.5 million or so customers out of service.

Buildings stood in the dark early Tuesday while traffic intersections were without stop lights, with only the headlights of vehicles lighting up the streets.

In a social media post, Puerto Rico Governor Pedro Pierluisi said officials were in communication with Luma and others on the major blackout affecting a large part of the island on New Year's Eve.

"We are demanding answers," he said, urging for the public to be informed on measures being taken to restore service throughout the island.

Luma said it would provide regular updates on the restoration process on social media.

The US took control of Puerto Rico, a Spanish-speaking archipelago, in 1898. The island's latest infrastructure issues, exacerbated by the devastating Hurricane Maria in 2017, come as many residents continue to demand their independence.