A fire at an electrical substation in Puerto Rico has knocked out power to hundreds of thousands throughout the island, utility company LUMA Energy said Thursday.

"The fire caused major blackouts across the entire island. The situation is under assessment and work is being done to restore the system," LUMA Energy tweeted, stating that restoration efforts will continue overnight.

About 400,000 customers in Puerto Rico were without power as of late Thursday, LUMA Energy said, which is more than 1 in 10 residents on the island.

The fire broke out in a transformer at LUMA's Monacillos substation, the company said. Personnel from the Puerto Rico Police, Fire Department and Medical Emergencies are at the scene and no injuries have been reported.

Calling it an explosion, Gov. Pedro Pierluisi said that both state and federal law enforcement authorities are investigating the cause.

Read more: CNN