Ohio ? An explosion at a power substation Monday in Columbus sent fireballs into the sky and knocked out electricity to about 35,000 customers.

Arson investigators said two transformers blew up due to a malfunction. The power company was investigating the cause.

Firefighters using foam had the blaze under control in about two hours.

Six families were forced from their homes while officials checked for dangerous fumes, and several schools that lost power were closed. The families were allowed to return home within an hour of the 5:30 a.m. blast at American Electric Power.

Many traffic lights were knocked out, slowing rush-hour traffic. No auto accidents were reported.

As of Monday evening, 14,000 customers remained without power, the utility said.