MIAMI >> More than 1.4 million customers in Puerto Rico lost electricity Wednesday when all of the island’s power plants were knocked out of service, the latest frustrating blackout for residents who have suffered years of them.

A preliminary review suggested the cause was a problem near a transmission line in southern Puerto Rico that began at 12:38 p.m., according to Josué Colón, the island’s energy czar. He said work to restore the power would most likely extend well into today.

“This process is not going to end today,” he said in a news conference Wednesday afternoon.

The blackout led to gridlock on Puerto Rico’s roads as transit officers tried to direct cars at intersections with disabled traffic lights. People rushed to gas stations for fuel to power emergency generators. An urban train in San Juan, the capital, came to a stop, forcing passengers to clamber out and walk down an overpass. Plaza Las Américas, the island’s biggest mall, largely shut down.

Puerto Rico has suffered extensive power grid problems since Hurricane Maria devastated the island in 2017, including periodic blackouts that have exasperated residents and businesses, forcing them to change or cancel plans and contend with losing appliances and electronics to unpredictable power surges.

The island often recovers slowly from major power disruptions. It has taken as long as several days to restore service after similar blackouts in the past, including an archipelago-wide outage on New Year’s Eve. That blackout, like the one Wednesday, included the smaller island municipalities of Vieques and Culebra.

Gary Soto, director of the energy management and transmission operation center for Luma Energy, the company that distributes power in Puerto Rico, said at the news conference that full restoration could take up to 72 hours. Daniel Hernández, vice president of operations for Genera PR, the company that generates power, said plants may come back online more quickly than in previous years because the company has invested in quicker restart systems.

Hospitals were running on generator power, according to Verónica Ferraiuoli, designated secretary of state, who is acting as governor in the absence of Gov. Jenniffer González-Colón. The governor was away on vacation off the island but “in constant communication,” Ferraiuoli said.

Many Puerto Ricans were preparing Wednesday to be off work starting today as part of Holy Week, which also draws many tourists to the island.

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.