Power is still out for around 850,000 customers in Puerto Rico on Friday, after a massive outage left 1.5 million customers without power, according to LUMA Energy, the company that took over transmission and distribution from the island’s power authority.

As of 8:30 a.m. on Friday, power was restored for more than 600,000 customers. LUMA said most of the island should have power by Friday night.

The power outage came after a fire erupted at one of the island’s four main power plants. The exact cause of the interruption of service is under investigation, LUMA said.

“The extent of the outage has impacted each generating facility in Puerto Rico and a significant effort to restore service is underway,” LUMA said in a statement.

Given how widespread the outage is, the government and the energy company said there is no timetable for full restoration.

“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,” LUMA said.

Public schools were closed to students on Thursday and Friday due to the outage, according to the island’s governor, Pedro Pierluisi.

Around 100,000 customers were also without water on Thursday due to the power outage, according to president of the Aqueduct and Sewer Authority, Doriel Pagán Crespo.

The outage also affected some traffic intersections by shutting off traffic lights, officials warned.

Only essential personnel are being summoned to central government agencies on Friday, Pierluisi said, in another tweet.

Addressing the upcoming hurricane season, Josue Colon, the executive director of the power company, said the electrical grid still needs to be reconstructed and that it would not be responsible for them to say that the island’s grid could withstand a hurricane like 2017’s category 5 Maria.