A major power outage plunged much of Puerto Rico into darkness on New Year’s Eve, with the US island territory’s electric utility saying restoration could take up to two days.

The “island-wide blackout” began at 5:30 am Tuesday (0930 GMT), Luma Energy, which manages power transmission on the island, said in a social media statement. It added that preliminary findings pointed to a fault in an underground cable as the cause of the outage – although the situation remained under investigation.

Jose Perez, director of external affairs at Luma, told AFP that the fault “caused a cascade effect” leading to the general blackout. “We have already begun the restoration process for some customers, and the entire process will take 24 to 48 hours,” Luma said.

US President Joe Biden was briefed on the outage, the White House said, noting that federal assistance was available if needed to speed up the restoration timeline. “This is a long and arduous process that requires local and federal collaboration,” Puerto Rico Governor Pedro Pierluisi said on X, adding that he requested the Federal Emergency Management Agency (FEMA) to “continue facilitating the projects we have begun.”

The blackout came as the Caribbean island entered its high season for tourists visiting from the wintry US mainland, and just hours ahead of New Year’s Eve celebrations.

Luma added in a subsequent statement that service had been restored to some critical facilities, including a medical center and hospital in the capital San Juan.

As of 7:00 pm local time (2300 GMT), the agency has restored service to more than 335,000 customers, approximately 23 percent of clients impacted by the outage, according to Luma’s emergency response dashboard.

The company earlier noted that the incident left its roughly 1.5 million customers without service.

“The truth is that we have been dealing with this problem for a while,” said Ismael Perez, a resident of Dorado, expressing frustration with the island’s power grid. He said he was concerned that residents could be left without power for more than two days.