A blackout affected the entire island of Puerto Rico on Wednesday — just one day after the region suffered an outage that affected 97 percent of customers.

The region was already suffering from an unstable power grid following Hurricane Maria, which barreled into the island seven months ago.

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

This is the first time the island has experienced a full island-wide blackout since the Category 4 storm hit on Sept. 20. Some 40,000 power customers were still without normal electricity service as a result of the hurricane.

The Associated Press contributed to this report.