Earlier on Friday, the grid operator said it hoped to have 1 million customers back online by the end of the day. "This is subject to safety -- for the system, for the employees and for the community," Kevin Acevedo, vice president of LUMA Energy, the operator of Puerto Rico's power grid operator, told reporters in San Juan.

The company was unable to commit to a timeline for the restoration of power to every customer, according to an announcement on its website.

"We are continuing to make progress in restoration but due to extensive damage at Costa Sur Substation, we are not in position to provide an estimate of full restoration at this time," the energy provider said.

An unspecified failure led to a fire at the plant outside the town of Guayanilla on the southwest coast around 8:45 p.m. Wednesday, cutting power across the island, Acevedo has said. Firefighters have since extinguished the flames.

All customers lost power initially, Josue Colon, Puerto Rico's lead telecommunications and infrastructure engineer, told reporters Thursday, "because all the generating units went offline."

The exact cause wasn't immediately known, the utility has said.

"Every single (piece of) equipment (at the plant's switchyard) needs to be inspected and tested to make sure that when it's back in service, we can restore power for customers reliably and safely," Shay Bahramirad, LUMA Energy's senior vice president of engineering and asset management, told reporters in San Juan on Friday.

The power outage also has interrupted water service to tens of thousands of homes and businesses, Gov. Pedro Pierluisi said, citing the island's aqueducts and sewers authority.

Puerto Rico canceled classes Friday for students for a second straight day. But school principals, custodians, and school cafeteria employees still were told to report to work Friday, the island's education department said.

The island's courts also were closed because of the outage, though some court services will be available online for urgent matters like mental health and restraining orders, the territory's judicial system said.

All hospitals were operational by Thursday afternoon, whether their power was restored or they were operating via a generator, according to Secretary of the Interior Noelia García Bardales.

Replacement power plant parts ordered, official says

Early Thursday, the utility said the "massive island-wide blackout" might have been "caused by a circuit breaker failure" at the Costa Sur plant.

Firefighters extinguished flames that affected two substations at the plant, the Bureau of Puerto Rico Fire Departments said Wednesday.

The cause of the fire is being investigated, Acevedo said Thursday morning, adding that the equipment was up to date on maintenance inspections.

Cleanup at the plant is underway, and replacement parts have been identified and ordered, Acevedo said.

LUMA is a joint venture of Quanta Services and the Canadian energy company ATCO, which the Puerto Rican government chose to take over the operation of the power grid from its previous public electric utility, the Puerto Rico Electric Power Authority. LUMA has been in charge of the power grid since June 1.