An explosion and fire at a power substation caused a blackout in parts of northern Puerto Rico on Sunday, according to authorities.
Puerto Rico's Electric Power Authority (PREPA) said on Twitter that 400 megawatts of power generation had been lost.
The fire was caused by a mechanical failure and impacted sections of San Juan, Trujillo Alto, Guaynabo, Carolina, Caguas, and Juncos, PREPA spokesman Carlos Monroig told CNN.
Monroig said the majority of customers would have power restored Sunday night, but that communities powered by substations Villa Betina and Quebrada Negrito would likely be without power until Monday afternoon.
No injuries were reported, San Juan Mayor Carmen Yulin Cruz tweeted on her verified account.
"It appears fire has almost totally been extinguished and the cooling phase is beginning," Yulin posted.
She told CNN that light was already beginning to return to several areas, but that she didn't know the percentage of people with or without power in San Juan.
It is also unclear how many people are with or without power across the affected areas of Puerto Rico.
Puerto Rico's 3.4 million citizens have endured months of hardship since Hurricane Maria made landfall on September 20 with 155 mph winds.
The Category 4 storm cut power to much of the island. In late January, PREPA issued a statement saying more than 450,000 customers remained without electricity.
The authority said then that it was working with state and federal agencies, including the US Army Corps of Engineers, FEMA, and power industry contractors, "to execute an aggressive plan to advance the restoration."
PREPA came under fire late last year after entering into a controversial $300 million contract with Whitefish Energy, a small Montana-based firm that employed two people when the storm hit.