An underground transformer fire in Co-op City left thousands without power Friday. 

Police sources say the incident began in the basement of building 8 around 10 a.m. FDNY crews responded to put the fire out. 

The power outage affected Section 1 in Co-op City, which is buildings 1-8. Census data shows that 9,000 residents live in those buildings, and have an average age of 55.

Police say power is slowly being restored to residents Friday night.