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SAN JUAN, Puerto Rico (AP) — The U.S. Virgin Islands’ power company on Friday reported a complete blackout in St. John and St. Thomas, which is the U.S. territory’s most populated island.

Officials said the outage occurred late Thursday when a key power plant lost all generation capacity for unknown reasons.

The Virgin Islands Water & Power Authority said it did not know when power would be restored. More than 51,600 people live in St. Thomas and more than 4,000 people live in St. John.