Much of northern Puerto Rico that had seen power finally restored months after Hurricane Maria, was in darkness again on Sunday following an explosion and fire at an electrical substation.

The Puerto Rico Electric Power Authority, or PREPA, says several areas are without power, after an explosion and fire at the substation in Monacillo on the outskirts of the capital, San Juan.

NPR's Adrian Florido, reporting from San Juan, says the blackout affects the heavily populated northern part of the island after the explosion that occurred at about 9 p.m. Sunday.

As of Sunday, a third of the utility's customers were still without electricity after losing it in the wake of Maria in September, Adrian says.

PREPA says it does not yet know what caused the explosion, but that it is optimistic that the substation could be repaired and that power could be restored within a day.

On Twitter, the utility said that electricity had been turned back on for San Juan's international airport and a medical facility.

As we reported in January, Puerto Rico's Gov. Ricardo Rosselló has announced a plan to privatize PREPA, which has been under fire for its failure to fully restore power to the island after the devastation of Maria, Puerto Rico's worst natural disaster in nearly a century.

That came after Rosselló asked the Justice Department to investigate the utility "after federal agents said they found large quantities of critical rebuilding materials stored in a warehouse owned by the public company."

Meanwhile, the governor announced Friday that Puerto Rico would get $16 billion in federal disaster aid signed by President Trump under a new spending plan.

In October, PREPA hired Whitefish Energy Holdings, a little-known Montana-based company with ties to Interior Secretary Ryan Zinke, to head up the effort to restore power. The $300 million contract drew intense criticism and in late October, Rosselló announced the contract with Whitefish was being terminated.

Copyright 2021 NPR. To see more, visit https://www.npr.org.