In this photo provided by WKOW-TV one of two fires as power substations burns in Madison, Wis., knocking out power to more than 11,000 customers on what is promising to be a sweltering day. (WKOW-TV via AP)
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(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.

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.

They include residents from the west side of downtown’s Capitol Square to areas around the Yahara River, Schultz said.

A second blaze occurred at a substation on East Campus Mall, about two miles southwest of the first, a statement from ATC said. That fire was contained, and its cause is unknown.

It’s possible the fires were connected, Keebler said.

The power outage comes as a potentially deadly heat wave hovers over the Midwest, prompting cities throughout the region to warn residents to stay hydrated and cool.

Madison on Friday morning was under an “excessive heat” warning, with high temperatures forecast to reach the mid-90s. The heat index will reach values of 105 to 110, according to the National Weather Service.

The fires don’t appear to have any correlation to the hot weather, according to Keebler. He said utility authorities believe the fire at the downtown substation was probably due to a mechanical issue.