Almost all power has been restored to customers in Mercer County after a transmission line issue left about 21,000 homes in the dark Saturday.

First Energy's outage map showed a lot of the outages were in the Hermitage area. 

21 News spoke with First Energy officials, who said the mass outage lasted about one hour and was caused by a problem in a 69,000-volt transmission line.

They said that line spreads to several substations in the area.

Officials are still investigating the exact problem involving the transmission line.