(Reuters) – Puerto Rico woke up to darkness on Tuesday morning after a power grid failure left almost the entire island without power.

Around 90% of customers were without power as of 9:30 a.m. (1:30 GMT), according to energy distribution company Luma Energy’s real-time portal.

It will likely take 24 to 48 hours for the lights to come back on, “conditions permitting,” Luma said in a statement.

“While the cause of the outage is under investigation, preliminary results indicate an underground line failure,” Luma added.

Ivan Baez, a spokesman for power producer Genera, described the grid failure as a “serious incident” in a local radio interview.

He said the suspected failed line was operated by Luma and brought down Genera power plants and private generators.

Genera’s website showed two plants restarting at 9:30 a.m. on US territory.

Puerto Rico has long struggled with chronic power outages as its infrastructure collapses. In 2017, it was hit hard by Hurricane Maria, a Category 4 storm.

(Reporting by Ivelisse Rivera and Kylie Madry; Editing by Kirsten Donovan and Sharon Singleton)