Around 90% of clients were without power at 09:30 (1330 GMT) on Tuesday, according to energy distribution company LUMA Energy’s real-time portal.

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

“While the cause of the outage is under investigation, preliminary findings point to a failure in an underground line,” LUMA added.

Ivan Baez, a spokesperson for power generator Genera, called the grid failure a “major incident” in a local radio interview.

He said that the line believed to have failed was operated by LUMA and brought down plants belonging to Genera as well as private generators.

Puerto Rico Governor Pedro Pierluisi said in a post on social media that his administration was “demanding answers and solutions” from LUMA and Genera.

He also said that the companies “need to speed up bringing back online the generator units … and keep the people informed about the measures they’re taking to bring service back to the whole island.”

Genera’s website showed two plants starting up again at 09:30 in the US territory.

Puerto Rico has long dealt with chronic power outages as its infrastructure crumbles. It was hit hard by Hurricane Maria, a Category 4 storm, in 2017.