Brutally cold temperatures gripped the US Midwest on Thursday, freezing water mains, causing power outages, cancelling flights and straining natural gas supplies.
Tens of millions of Americans shivered for a second day as the mercury dipped to record lows in several states.
More than a dozen deaths have been attributed to the sub-zero weather and a weekend snowstorm that blanketed the same region. Many of the cold-related fatalities were in Michigan, where the governor said the death toll was still being confirmed.
Schools and businesses remained closed in several midwestern states, people were encouraged to stay home, and travellers were stranded by grounded flights and halted trains.
In Michigan and Minnesota, natural gas supplies were under threat.
Authorities asked residents to reduce heat consumption wherever possible and decreased heating in government buildings.
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Water mains froze in Detroit, Chicago and parts of Canada, and power outages were reported in Iowa and Wisconsin.
Nearly 1,700 flights had been cancelled in Chicago by Thursday afternoon. Airport crews worked in 15-minute increments on the tarmac to avoid frostbite.
Rail service Amtrak planned to begin partially restoring service after cancelling all lines Wednesday in and out of Chicago.
The deadly, sub-zero temperatures were expected to lift Friday, but the misery would not end quickly in the roughly dozen states most affected.
"We are not done yet. We've got another 24 hours where the weather will be at dangerous levels," Michigan Governor Gretchen Whitmer told a news conference.
The National Weather Service (NWS) said temperatures would slowly moderate, but the agency forecast wind chills Thursday would remain between -20 to -50 degrees Fahrenheit (-29 to -46 Celsius) over parts of the Upper Midwest.
The cold has frozen sections of Niagara Falls and sent blocks of ice floating down the river winding through downtown Chicago.
The Arctic air mass that descended from its usual northern rotation on Wednesday caused the second coldest day ever recorded in the Windy City, where residents reported hearing "frost quakes."
"With more snow on the way, we're not out of the woods yet."