(CNN) About 12,000 customers are without electricity in Madison, Wisconsin, during a dangerous heat wave, after fires at two downtown substations, according to local officials.

An explosion and a fire occurred early Friday morning at a substation jointly owned by Madison Gas and Electric and the American Transmission Company, MG&E CEO Jeff Keebler told reporters at a news conference.

The equipment needed to be turned off so firefighters could put out the fire, Keebler said.

Smoke from fire at Madison Gas and Electric on the East Side #Madison #Fire pic.twitter.com/sUcMA34Vt1 — Erin (@Ern608) July 19, 2019

Officials are assessing damage to see when power can be turned back on, Keebler said. They currently have no estimate.

MG&E spokesman Steve Schultz initially told CNN that more than 13,000 customers were without electricity due that fire. Keebler later estimated 12,000.