An underground electrical fire inside a manhole knocked out power for much of downtown St. Louis Thursday, leaving many buildings without air conditioning on a day when the temperature was expected to reach triple digits.

City Hall, an adjacent building known as City Hall West and the Carnahan courts building were among those closed for the day due to the outage that began after the fire broke out Wednesday night. The city police department building remained open but the city health department, administrative offices and municipal courts were all closed for the day, said Maggie Crane, spokeswoman for Mayor Francis Slay.

Utility company Ameren Missouri said a fire inside a manhole at Seventh and Market Street caused outages that began around 8:30 p.m. Wednesday. The cause was not immediately known, but Ameren spokeswoman Trina Muniz said aging infrastructure combined with extreme temperatures likely caused the equipment failure. Temperatures have been in the upper 90s for several days in St. Louis, with forecasters predicting a sweltering high of 100 degrees on Thursday, the fifth straight day when the heat index was expected to top 100 degrees.

The fire, which reached a temperature of about 400 degrees, was out by midnight, but it was so hot inside the manhole that workers couldn't get inside to make repairs, delaying the restoration of power. Several circuits were inside the manhole.

Power was expected to be restored by Thursday afternoon, Ameren said.

The outage affected about 2,100 customers, mainly comprising homes and offices in high-rise buildings. Many businesses sent workers home or to alternative locations for the day.

The city said the affected area runs from Sixth Street to 20th Street east and west, and Spruce to Olive south to north. Most traffic lights were also not functioning as of Thursday morning.

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Information from: St. Louis Post-Dispatch, http://www.stltoday.com