SAN JUAN, Puerto Rico — Power went out in Puerto Rico’s capital Friday for the second day in a row as the U.S. territory struggles to restore electricity to more than 200,000 people who remain in the dark nearly six months after Hurricane Maria.

The island’s Electric Power Authority tweeted that crews repaired a failure at a substation in San Juan but provided no other details. It was unclear what caused it or how many customers were affected. Power company officials did not return messages for comment.

Fredyson Martinez, vice president of a union that represents Puerto Rico power company workers, told The Associated Press that it appeared the problem was caused by a switch failure.

“The system is unstable,” he said. “This is reflected in the substations and everything else.”

On Thursday, some 800,000 customers were left in the dark after two of the island’s main power stations shut down when a transmission line failed. It was unclear whether power had returned to all those customers. Officials said Thursday’s outage affected the municipalities of San Juan, Bayamon, Carolina and Caguas, although people on social media reported outages in other municipalities as well.

A backup power line in the area of that failure has not been repaired since Hurricane Maria hit on Sept. 20 as a Category 4 storm, damaging two-thirds of Puerto Rico’s power distribution system.

Shortly before Friday’s blackout, power company director Justo Gonzalez said in a statement that electricity had been restored to nearly 90 percent of the island’s electricity customers. He said the government would receive more materials in the coming weeks for the power restoration effort, including nearly 250 transformers, more than 7,400 electrical posts and more than 1,000 miles of cable.

He also said several damaged transmission lines would be repaired in upcoming weeks and that would help stabilize the system.

“Although the effort has been great, we are still not satisfied and we will not rest until we energize all of Puerto Rico,” he said.

The U.S. Army Corps of Engineers has said it expects Puerto Rico will be fully energized by May.