By Veronica Calderon, CNN en Español

(CNN) — As Puerto Ricans welcomed 2025, many faced the grim-reality of celebrating in the dark as outages lingered following another massive failure of its power grid that impacted the majority of the island Tuesday.

The outage stemmed from long-lasting structural issues with both generation and distribution, involving GeneraPR, one of the companies operating the power plants, and LUMA Energy, the Canadian-American company responsible for transmission on the island.

While the cause of the outage is still under investigation, it appears there was an issue with an underground line, LUMA said.

Josue Colon, the CEO of Puerto Rico Power Authority (PREPA) said there was a breakdown in the transmission and distribution system that triggered a series of events that, in turn, caused all the generating units to stop operating. PREPA is a government-owned corporation that controls all the electricity generation, transmission and distribution properties in the island, and contracted both LUMA and Genera to operate separate segments of its electricity business.

Power outages on the island have been a long-running source of frustration for Puerto Ricans who rely on a fragile and poorly-maintained power grid, with modernization efforts slow to materialize over several decades, first by a highly indebted PREPA, and today by private operators.

The massive blackout began on New Year’s Eve at 5:30 a.m., affecting a staggering 90% of customers, according to a post on Gov. Pedro Pierluisi’s X account.

As of 9 p.m. ET, a little over 765,000 users, or more than 50% of customers, were still without power, but service was restored at hospitals and other major facilities, including the water and sewer authority, the company said.

By 5 a.m. Wednesday, service was back for 1,128,358 customers or 76.9% of the total, LUMA reported.

“We can report that work is already underway to restore service with the San Juan and Palo Seco plants,” Pierluisi said.

“We are demanding answers and solutions from both LUMA and Genera, who must expedite the restart of the generating units outside the fault area and keep the people duly informed about the measures they are taking to restore service throughout the Island.”

With memories of Hurricane Maria in 2017 and recent earthquakes still fresh, locals are questioning the electric company’s ability to provide reliable service.

Amid rising electricity prices and a federal oversight board in place for nearly a decade, Puerto Rico residents like Carlos De Leon – whose refrigerator remains down – are increasingly turning to personal generators.

“Most of the island is without power,” De Leon told CNN. “The electric company hasn’t been able to do what they were contracted to do. And that was to give us a good power service. … And to have this big failure that began at dawn, it’s just crazy.”

“These companies just care about the profit. They don’t care about the people. They are not half of what our public service used to be. The electric company was a public service and now it’s privatized, and they don’t care about the problem. They say they’re sorry, but they’re not really sorry,” he added.

LUMA Energy’s Emergency Operations Center is working with Genera and other power collaborators to restore the electrical system and power to the island as quickly as possible, the company said. The full restoration process will take about a day or two, the company added.

President Joe Biden spoke with Pierluisi Tuesday “to discuss the widespread power outages across Puerto Rico and offer any federal assistance that they need,” the White House said. Biden also spoke with Energy Secretary Jennifer Granholm, “and directed her to continue to offer any assistance the Biden Administration can provide to speed power restoration for the people of Puerto Rico.”

Residents expressed their displeasure with the outages on Tuesday ahead of 2025 celebrations.

“Complete frustration,” Sonia Arroyo, a resident of Vega Alta outside of San Juan, told CNN en Español. “Frustration, disappointment with the government, with the institutions, with everything.”

“I am more concerned about the development of this country than about the party,” said Liz Rebeca Aponte of Dorado. “Here we work in the pharmaceutical industry, and industries that want to invest in the country will not come here if we do not improve the structure.”

The New Year’s Eve outage is just the latest time that Puerto Rico’s power system has faltered on a mass scale following decades-long of structural problems marred such as high debt and low maintenance and investments.

The collapse of the power grid in 2017 after Hurricane Maria left hundreds of thousands of people without power for months while both the territory’s government and Prepa were tied on a difficult renegotiation of their mounting debt with creditors and the federal government. That outage was considered the largest blackout in US history in terms of the total number of lost hours of electricity.

Since then, the US Federal Emergency Management Agency has awarded $9.9 billion for permanent projects to repair damage caused by Hurricane Maria. LUMA Energy took over management of the grid in 2021 from PREPA. GeneraPR is in charge of energy production since 2023.

Still, the challenges remain. After Hurricane Ernesto in August, about half of all electric customers on the island were at one point without power, according to LUMA Energy.

This story has been updated with additional information.

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CNN’s Taylor Romine, Eric Levenson, Emma Tucker and Hanna Park contributed to this report.