More than a million people in Puerto Rico were still without power on Thursday after a fire at a primary power plant triggered the largest outage in the U.S. territory this year, forcing schools and government buildings to close.

The blackout also cut off water to nearly 160,000 people and hampered traffic across the 3.2 million-strong island, where the sound of generators and the stench of diesel filled the air. Those who couldn’t afford generators and had medical issues that required refrigerated insulin, such as diabetes, were concerned about how long they’d be without power.

The blackout could have been caused by a circuit-breaker failure at the Costa Sur generation facility, one of four primary plants on the island, according to Luma, the firm that took over transmission and distribution from Puerto Rico’s Electric Power Authority last year.

“The system is gradually being restored,” said Kevin Acevedo, Luma’s vice president, adding that the company is aiming to finish the job within 24 hours. “The people of Puerto Rico must realize that this is a system that has existed for a long time.” The task of restoring Puerto Rico’s system is sensitive and difficult.”

The specific cause of the outage, according to Luma, is unknown.

“It will necessitate a thorough inquiry,” Acevedo added. He claimed that the equipment that caused the fire had been properly maintained.

At least three generation units were back online by Thursday, according to officials, with engineers attempting to restore more.

The interruption occurred two months before the Atlantic hurricane season begins, raising concerns about Puerto Rico’s electrical grid’s state.

“Yes, the system is weak, no one denies that,” Acevedo added, “but we’re prepared.”

On Thursday, police officers were stationed at major junctions to assist with traffic flow, while health officials checked in at hospitals to ensure generators were still operational.

The interruption upset Puerto Ricans who were already angry over a power grid that had been destroyed by Hurricane Maria in 2017. Emergency repairs were done at the time, but reconstruction efforts have yet to begin, and power company officials blame the outages on aged, poorly maintained infrastructure.

The Costa Sur facility was also damaged by a series of powerful earthquakes that slammed southern Puerto Rico, where it is located.

In order to emerge from a lengthy bankruptcy, the Electric Power Authority is also attempting to restructure $9 billion in public debt. Corruption, incompetence, and a lack of upkeep have plagued the corporation for decades.

A major fire at a substation in San Juan, the capital, left hundreds of thousands without electricity in June of last year. In September 2016, a fire at a power facility caused an island-wide blackout.