Karina Santiago Gonzalez works on a small electricity generator in Morovis, Puerto Rico, in December 2017. (Carlos Giusti/AP)

About 840,000 people across Puerto Rico were without power Thursday as the U.S. territory continues to struggle after the damage caused by Hurricane Maria last fall, according to island’s Electric Power Authority.

The power company, in tweets Thursday, said that a tree had fallen onto the main line supplying power to the capital, San Juan, and its surrounding communities along the northern coast, and to the southeast part of the island. The tree fell as crews were clearing land in Cayey, a mountain town in southeastern Puerto Rico, as part of a power-restoration project connected with the hurricane.

The blackout is the latest source of frustration for Puerto Ricans, who have dealt with the power lines failing multiple times in recent months. Since the hurricane tore from the island’s southeast corner though the central mountains and out the northwest coast six months ago, several communities near Cayey remain without power. Across the rest of the island, more than 1,200 generators provided by the Federal Emergency Management Agency are still the primary source of power for hospitals, more than two dozen police and fire stations, correctional facilities and water pumps.

Blackouts are common: One in mid-February caused by an  explosion at an electrical substation left nearly 1 million around San Juan without power. What is worse is that the next hurricane season is less than two months away.

Power slowly began returning to various parts of San Juan on Thursday afternoon, said the capital’s mayor, Carmen Yulín Cruz, in a tweet. Backup generators powered Puerto Rico’s main public hospital and international airport.

The power company’s interim director, Justo Gonzalez, estimated that full restoration of power lost in this blackout could take up to 11 hours across the island.

“This is the SAME line that was ‘fixed’ by Whitefish,” the mayor tweeted, referring to Whitefish Energy, a small Montana firm that landed Puerto Rico’s biggest contract to restore power.

The contract was canceled after lawmakers began asking how the company, which had just two employees at the time of the storm, claimed it brought hundreds of workers and 2,500 tons of equipment to the island to make electrical repairs.

On Wednesday, federal officials told a congressional hearing that they hope to implement a plan by June to strengthen and stabilize Puerto Rico’s electrical grid, according to the Associated Press. The Atlantic hurricane season begins June 1. The initial estimated cost to rebuild the grid is $17.6 billion, although that number could change.

Bruce Walker, an assistant secretary with the Energy Department, said at the hearing that officials are exploring renewable energy sources and building microgrids around industrial sites, the AP reported. Puerto Rico depends on imported fuel, and manufacturing makes up 50 percent of its economy.

Crews also have installed concrete poles and made other improvements with the goal of bolstering the system.

“Those are very, very quick wins,” Walker said, according to the AP. “You don’t just build a grid overnight.”

The federal government will bear 90 percent of the cost for permanent work on the power grid, according to a top FEMA official, the AP reported. On Tuesday, the Department of Housing and Urban Development announced that as much as $2 billion of $18.5 billion in disaster recovery grants intended to rebuild communities on the island will also go toward the grid.

 Read more: