More than 1.4 million customers in Puerto Rico lost electricity on Wednesday afternoon when all of the island’s power plants were knocked out of service.

The cause was not immediately clear, according to the private electrical companies that generate and distribute Puerto Rico’s power.

The island often recovers slowly from power disruptions. It has taken as long as several days to restore service after similar blackouts in the past, including an islandwide outage on New Year’s Eve.

“We have experienced a massive blackout across the entire island due to all of the power generation plants unexpectedly going offline,” Genera PR, the company that generates power in Puerto Rico, said in a social media post on Wednesday.

Luma Energy, the company that distributes energy, said in its own post that the blackout began around 12:40 p.m. on Wednesday. Many Puerto Ricans were preparing on Wednesday to be off work starting on Thursday as part of Holy Week; Gov. Jenniffer González-Colón was away on a personal trip.

Though Luma’s website showed that about a third of customers had power more than an hour later, Hugo Sorrentini, a Luma spokesman, said it was unlikely that any customer actually had any electricity, because all the power plants were offline.

In an interview with WKAQ-AM, a local radio station, shortly after the blackout began, Josué Colón, the island’s energy czar, said that it was too early to know what had set off the systemwide shutdown.

“We don’t know the cause,” he said, adding that restoring power would probably take at least a day.

Mr. Colón had warned in a radio interview last month that Puerto Ricans were likely to experience summer blackouts because the island did not have enough generating capacity available to meet peak summer demand.

That warning prompted three Democratic members of Congress — Ritchie Torres of New York, Darren Soto of Florida and Pablo José Hernández, Puerto Rico’s nonvoting resident commissioner — to send a letter to the Trump administration on Tuesday, raising the alarm “about the imminent grid reliability crisis confronting Puerto Rico’s electric system.”

According to the letter, the island is projected to experience power shortfalls on 90 days between June and October.

Puerto Rico has suffered extensive power grid problems since Hurricane Maria devastated the island in 2017.