Power has been restored to most of Puerto Rico after a major blackout plunged the island into darkness on New Year's Eve, the US territory's electric utility said.
Electricity supply had been restored to at least 1.25 million customers, or 85.6% of those affected, by 10:00am local time (1400 GMT), according to Luma Energy.
The power company said preliminary findings indicated a fault in an underground cable may have been the cause of the outage, although investigations continue.
Puerto Rico, a Spanish-speaking archipelago, came under US control in 1898 and has faced perennial infrastructure issues, exacerbated by devastating hurricanes and a 2020 earthquake.
The utility company said power had been restored to both of the capital San Juan's major airports, key water supply facilities and to at least 31 hospitals.
Governor Pedro Pierluisi said he had spoken with US President Joe Biden, and had asked for continuing Federal Emergency Management Agency (FEMA) support.
"This is a long and arduous process that requires local and federal collaboration, which is why I requested that FEMA continue facilitating the projects we have begun," he said in a social media post.
He said Washington had approved a two-year extension for the use of temporary generators that provide the island 350 MW in energy, which will now continue to do so until 2027.
Those generators had been put in place following the impact of Hurricane Fiona in 2022.
Earlier, Luma's director of external affairs Jose Perez said the fault that initiated the power failure had caused a cascade effect that then led to the general blackout.
On Tuesday, Luma said it could take up to 48 hours for full restoration of power to all customers.
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.
Criticism of power grid operators
Ivan Baez, a spokesperson for power generator Genera, said in a local radio interview that the line believed to have failed was operated by LUMA and brought down plants belonging to Genera, as well as private generators.
LUMA did not answer direct questions about responsibility for the power lines.
Ramon Luis Nieves, an attorney in San Juan and a former senator on the island, said that New Year's Eve is typically a time for family reunions and watching fireworks.
He said that he is not surprised by the outage, especially after Hurricane Maria, a Category 4 storm that hit the island in 2017.
Mr Nieves has long been a critic of operators of the power grid.
"This disaster has been more than a decade in the making," he said.
"The (power) generators are old, long past their life expectancy and the operators have failed to properly invest for years," he added.
Such sentiment is common on the Caribbean island, a US territory whose residents are US citizens but do not have voting representation in Congress and cannot vote in presidential elections.
Protesters have called for the island's government to cancel its contract with LUMA.
In response to a 2022 protest, LUMA said it had "inherited an electrical system that suffered years, in fact decades, of abandonment".
Jenniffer González, who takes office as governor of Puerto Rico tomorrow, said on social media that she is forming an energy taskforce to address the frequent blackouts.
"We cannot continue with an energy system that fails our people so often," she said.
"Events like this morning's blackout and the uncertainty of a prompt restoration of power to the island continue to affect our economy and quality of life," she added.
