By Leyla Santiago, Khushbu Shah and Elwyn Lopez, CNN

(CNN) -- Power was gradually returning to San Juan, Puerto Rico, on Thursday after a massive outage struck the city.

The problem was with a failure on a main north-south transmission line, said Fernando Padilla, an official with the Puerto Rico Electric Power Authority, known as PREPA.

"It was a mechanical issue on the line, could have happened at any line," he said. "It's being patrolled and repaired by PREPA."

Power went out about 11:30 a.m. local time (10:30 a.m. ET) and was repaired Thursday night, said Ricardo Ramos Rodriguez, executive director of PREPA.

PREPA spokesman Carlos Monroig said 25% of power was back and he expects 42% of power to be restored by Friday morning. He said he didn't know what caused the transmission line to fail.

Homes and businesses that had been on grid power had to restart generators if they had them.

Marianne Sanchez told CNN her Rio Piedras neighborhood got power back just six days ago. She wrote on Instagram, "This is a not too friendly reminder of how fragile our power system is."

Power generation plummeted from over 40% of capacity to 18% on Thursday, PREPA said.

Much of Puerto Rico has been without power since Hurricane Maria smashed the island on September 20.

The-CNN-Wire

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