One of the first signs of trouble arrived at 9:41 p.m. Eastern time on Thursday, when most of Wall Street was shut and traders were still enjoying the Thanksgiving holiday in the US.

“Due to technical issues,” CME Group Inc. said in a one-line email to clients, its futures and options “markets have been halted.”

The problem, it turned out, was the cooling system at a data-center complex in the suburban town of Aurora, Illinois, some 50 miles (80 kilometers) from Chicago, which serves as the main hub for trillions of dollars of derivatives traded each day. Inside, temperatures soared past 100F (38C) despite the frigid weather, according to people familiar with the matter.

At the time, the CME’s information indicated the outage at the facility — run by private-equity owned CyrusOne — woul

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