The outages were “caused by extreme heat, cable failure and routine maintenance — all combining causing system overload,” Robert Warfield, spokesman for Duggan, told CBS affilate WWJ.

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Around 9:30 a.m., a cable feeding a substation failed and the city’s Department of Public Lighting moved customers to a functional circuit breaker, which then triggered the blackout, Duggan told reporters.

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By 2 p.m., one-third of those customers had had power restored, with 70 percent of them expected to have power by early evening and the remaining to get power by tonight, Jerry Norcia, president of Detroit power company DTE Energy, said Tuesday.

The Detroit Free Press reported that a high-profile murder trial was interrupted when the lights went out, and people evacuated the courthouse. Kenneth King, the presiding judge in the trial, told WWJ that an unreliable power grid is a security concern. “That’s a whole different dimension in very scary proposition,” King said. “Now, because we can’t use the elevator, the deputy sheriffs have to escort the prisoners down a dark stairway.”

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There were also reports of people stuck in elevators.

Schools let out early, reported Fox affiliate WJBK. City Hall was evacuated and the City Airport’s power also went out, reported ABC affiliate WXYZ. The Detroit People Mover transit system also lost power.

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Firefighters used radio to communicate with each other and firetrucks were moved out of garages so they could respond to calls as all stations lost power, officials said Tuesday. The 911 dispatch center never lost power, and police departments relied upon a back-up system, officials said.

A DTE Energy outage map showed the blackout was concentrated in downtown Detroit. WJBK reported about 100 customers were affected by the outages, and they were non-residential, public buildings.

Back-up power kept the lights on at Wayne County Jail, and DMC Detroit Receiving Hospital was running on partial power, the Detroit News reported.

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The lights went out as Detroit’s hockey team, the Red Wings, were practicing.

Similar major outages hit downtown Detroit in 2013, which also affected the Frank Murphy Hall of Justice and the People Mover. The city had to shut off power to a major portion of the power grid to fix a cable failure.

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As part of its bankruptcy process, Detroit is transitioning from running the PLD to having DTE Energy run the grid — a handover that will take four years, Duggan said.

Over the summer, DTE Energy focused on inspecting the most error-prone circuits, with the inspection process expected to be completed next year, Duggan said. The circuit that failed this morning hadn’t been addressed yet, “but obviously it’ll become the focus of inspections right now,” the mayor said.

Inspecting circuits “will make an event like today far less likely to occur again,” Duggan said. “We will, for another few years, have some risk of this happening.”