An island-wide blackout struck Puerto Rico on April 18, plunging the U.S. territory of more than 3 million citizens back into darkness. (Video: Reuters)

An island-wide blackout struck Puerto Rico on Wednesday, plunging the U.S. territory of more than 3 million citizens back into darkness, more than seven months after Hurricane Maria demolished its fragile power grid.

The Puerto Rico Electric Power Authority said the blackout, which began at about 11:30 a.m., could last from 24 to 36 hours. At about 5 p.m., the local utility said more than 51,000 customers had power.

In a statement Wednesday evening, Gov. Ricardo Roselló said the outage was caused by a subcontractor for energy company Cobra when workers damaged a major distribution line while moving a collapsed tower. PREPA said that the distribution line connects a major generating station in the south of the island to substations in the north, where most of the population lives.

“I have suggested to the PREPA Board of Directors that they cancel the contract with the Cobra subcontractor who is directly responsible for this power outage,” Roselló said in the written statement.

Maria “Mayita” Melendez of Ponce, a large city on Puerto Rico’s southern coast, said PREPA told her that workers for Cobra Energy were moving the tower near Aguirre, one of the island’s major generating plants, when an excavator struck the line, causing a change in voltage that created a chain reaction.

In a video statement Wednesday afternoon, PREPA interim director Justo Gonzalez said that when the line failed, the central generating plant in the south stopped functioning and seven major plants along the southern coast collapsed one by one.

The agency said its priority is to restore service to critical structures such as hospitals, the airport, water pumps and banks. PREPA tweeted that several hospitals and medical centers had power by midafternoon.

The power outage comes two days after PREPA published an online video celebrating the restoration of electric service to 97 percent of its customers.

Wednesday’s blackout was the first island-wide power outage since the storm.

In recent months, the island has experienced small, regional blackouts as well as wide-ranging outages, such as one last week that left nearly 1 million people without power. Last week, the agency blamed a fallen tree for an outage that cut power to 900,000 people.

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Tens of thousands of Puerto Ricans living in communities in the island’s east and central regions had yet to see their lights come back on since the hurricane. The restoration campaign has been plagued by political power struggles, rough terrain and logistical nightmares that have slowed and frustrated the populace and local leaders.

Mayor Angel Pérez Otero of Guaynabo, a jurisdiction near San Juan on the northern coast of the island, said his office was inundated by calls from residents about the blackout.

“They are having flashbacks to what they have already lived,” Pérez Otero said. “We’ve experienced too many of these, and frustration. It’s worrying how fragile our power grid is.”

In his city, Pérez Otero said a few businesses were operating only because their generators were still working. The backup power failed at a shopping center, but people continued to shop and use cash. Others rushed to gas pumps anticipating that they would close early and leave them without fuel.

But other parts of the city were “paralyzed,” he said.

The system’s fragility is especially concerning with the beginning of hurricane season only a couple of months away, giving federal and local officials little time to begin strengthening the decades-old and unkempt power grid in Puerto Rico.

“Sadly, this is the reality we will have to endure if another storm comes,” he said. “There’s no time to improve the system.”

The mayor said he hopes funding that Congress set aside for the island can be disbursed quickly and focused on mitigation plans to help residents make it through another storm season.

“It needs to start,” he said. “We are grateful for the companies and workers who came here. I would’ve liked if things went faster, but here we are.”