An explosion at a main power plant in Puerto Rico triggered a blackout across part of the island on Sunday, as officials said several municipalities were without power.

It was not immediately known what caused the explosion, which set off a fire, but areas of the U.S. territory — including parts of the capital city, San Juan — were without power.

“We are trying to restore that as quickly as possible,” the company told The Associated Press.

San Juan Mayor Carmen Yulín Cruz tweeted updates on the outage, reassuring the island that no injuries have been reported.

Around 9 p.m., Cruz stated that areas of San Juan — such as Placita and Santurce — have been getting their power turned back on. Just before 10 p.m., the mayor tweeted: “It appears fire has almost totally been extinguished and the cooling phase is beginning. No injured reported.”

More than 400,000 power customers have remained in the dark more than five months after Hurricane Maria slammed Puerto Rico.

The Category 4 storm destroyed two-thirds of the island's power distribution system and triggered up to an estimated $94 billion in damage.

Puerto Rico Gov. Ricardo Rosselló recently announced that he planned to privatize the state-owned power company, which is $9 billion in debt and relying on infrastructure nearly three times older than the industry average. It would be the largest restructuring of a public entity in U.S. history.

The Associated Press contributed to this report.