Puerto Rico was plunged into darkness early Tuesday morning by a nearly island-wide blackout.

The cause of the blackout is under investigation, but preliminary findings pointed to a fault in an underground line, according to Luma Energy, the island's main power distributor. Fully restoring service could take between 24-48 hours, the company said on X.

Only 13 percent of the island's 1.4 million customers had power as of about 1000AST (1400GMT), according to the New York Times.

An hour later, power had been restored to some areas, as well as San Juan's municipal hospital, Luma said.

The New Year's Eve blackout prompted renewed calls from elected officials and residents to address the unincorporated US territory's ongoing power issues, which have persisted since Hurricane Maria in 2017.

The island cannot continue to put up with an energy system that fails its citizens so often, Jenniffer González-Colon, Puerto Rico's current US congressional representative and the incoming governor of Puerto Rico, wrote on X.

Blackouts continue to affect Puerto Rico's economy and quality of life, she said.

On Facebook, the current governor, Pedro Pierluisi, demanded answers and solutions from the two main power companies, Luma and Genera.

Hundreds of thousands of residents at a time have been affected by power outages this year. A June outage left about 350,000 customers without power as temperatures climbed, and more than 700,000 customers lost electricity after Hurricane Ernesto in August.

As they awoke to another day without power, Puerto Ricans expressed frustration to US media.

"They're part of my everyday life," Enid Núñez, 49, said of the outages to the Associated Press.

Puerto Rico's power grid was strained even before Hurricane Maria devastated the island. US government funding helped shore up the grid, facilitate recovery projects from other natural disasters, and make other important infrastructure improvements.

But the implementation has been incomplete due to a variety of factors, such as issues starting construction and the Federal Emergency Management Agency's requirements to authorise use of some of the funds, according to a February 2024 report from the US Government Accountability Office.

"Inexcusably the power grid has still not recovered from damage in Hurricane Maria," Mark Levine, New York City's Manhattan borough president, wrote on X.

New York City is home to the largest Puerto Rican community in the mainland US.

"This is 3.5M American citizens," he wrote. "We owe them so much better."