A big fire erupted at an electricity plant that powers most of Puerto Rico on Wednesday, and a blackout swept across the U.S. territory of 3.5 million people.

The Electric Power Authority said two transmission lines of 230,000 volts each failed for reasons still being investigated. Spokeswoman Yohari Molina said in a phone interview that it was unclear when power would be restored.

The island's fire department said it extinguished a blaze at the power plant in southern Puerto Rico that serves a majority of customers on the island. Heavy storms were affecting the area where the plant is.

Authorities said the outage caused 15 fires across Puerto Rico as a result of malfunctioning generators, including at the upscale Vanderbilt hotel in the popular tourist area of Condado and at the mayor's office in the northern coastal town of Catano. All those fires were put out and no one was injured, officials said.

The blackout snarled road traffic across the island and the government had to cancel train service in the capital of San Juan and close a busy traffic tunnel in the island's southern region. Businesses, universities and government offices shuttered early, creating even more chaos on roads.

"I call on people to cooperate and respect drivers and government officials ... to avoid any regrettable incidents," Transportation Secretary Miguel Torres said.

The outage angered many Puerto Ricans who are struggling with power bills that are on average twice that of the U.S. mainland. People took to social media to demand where exactly their money is going.

The power company has faced numerous allegations of corruption and is struggling with a $9 billion debt that it hopes to restructure. Company officials have said they are seeking more revenue to update what they say is outdated equipment.

Puerto Rico is mired in a decade-long economic slumps and the territory's government is working to restructure nearly $70 billion in public debt that the governor has said is unpayable.