An island-wide blackout hit Puerto Rico Wednesday as the island struggles with an increasingly unstable power grid nearly seven months after Hurricane Maria hit the U.S. territory.

Electric Power Authority spokeswoman Yohari Molina told The Associated Press that crews are investigating the cause. Officials told the agency it could take 24 to 36 hours to fully restore power.

It’s the first time since Category 4 storm hit on Sept. 20 that Puerto Rico has experienced a full island-wide blackout.

Some 40,000 power customers were still without normal electricity service as a result of the hurricane.