Wednesday's outage came seven months after Hurricane Maria destroyed much of the island's electrical infrastructure and forced the entire US territory into darkness.
Mammoth Energy, the parent company of Cobra, said in a statement to ABC News "Cobra is dedicated to the hard work that lies ahead and continues to work around-the-clock with PREPA and the citizens of Puerto Rico to fix the entire infrastructure system to prevent outages such as this one from affecting the entire population on the island". Rosselló is pushing a bill to privatize the power system. Backup generators roared to life at the island's largest public hospital and at its main global airport, where officials reported no cancellations or delays.
Electric Power Authority spokeswoman Yohari Molina tells The Associated Press that crews are investigating the cause.
The priority is to restore service to critical structures such as hospitals, the airport, water pumps and banks, according to the agency's Twitter account.
The new blackout occurred as Puerto Rico legislators debate a bill that would privatize the island's power company, which is $14 billion in debt and relies on infrastructure almost three times older than the industry average.
On Wednesday evening, PREPA announced that more than 51,000 customers have electric service, though Puerto Rico has more than 1 million households.
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Tens of thousands of people have not had electricity since the catastrophic Hurricane Maria hit the island last September.
The same subcontractor was also blamed for an outage last week to 870,000 customers triggered by a tree falling on a power line, according to Justo Gonzalez, deputy director of PREPA. It was the second widespread failure in less than a week, underscoring just how fragile Puerto Rico's electricity remains since the storm hit. Just a month before Hurricane Maria struck a year ago, the board sued Rossello for refusing to implement proposed furloughs and pension cuts.
The United States Army Corps of Engineers has spent $2.1 billion on restoring power on the island, where 80 percent of the distribution lines were damaged during the hurricane.
With a debt of $9 billion, PREPA filed for a form of bankruptcy past year.
People wait to enter Hiram Bithorn Stadium for the final game of a two-game Mayor League Baseball series between the Minnesota Twins and the Cleveland Indians in San Juan, Puerto Rico, April 18, 2018.
On broader economic matters, the board wants Puerto Rico's government to make a Christmas bonus voluntary for private employers and allow businesses to dismiss workers without first having to prove just cause.