A blast of icy polar air brought dangerously low temperatures to the U.S. Midwest on Wednesday, causing at least three more deaths while halting mail delivery and forcing residents who pride themselves on their winter hardiness to huddle indoors.

At least a dozen deaths related to extreme cold weather have been reported since Saturday in Michigan, Iowa, Indiana, Illinois, Wisconsin and Minnesota, according to officials and media reports.

Nearly 7,000 Minneapolis area residents were without power on Wednesday. Xcel Energy has asked customers to dial down their thermostats to 60 F because of the strains on its natural gas supply system.

The Michigan Agency for Energy said the state’s utility companies had agreed not to shut off gas or electric supplies to delinquent customers for the rest of the week.

Wind-chill temperatures in parts of the Northern Plains and Great Lakes plunged as low as minus 42 F in Park Rapids, Minnesota, and to 31 degrees below zero F in Fargo, North Dakota, according to the National Weather Service .

The frigid winds began to blow into the U.S. East Coast later on Wednesday, sending temperatures plunging there.

The bitter cold was caused by a displacement of the polar vortex, a stream of air that normally spins around the stratosphere over the North Pole but whose current was disrupted and was now pushing south.

Officials opened warming centers across the Midwest, and in Chicago police stations were open to anyone seeking refuge.

Five city buses were also deployed to serve as mobile warming centers for homeless people, while city police handed out hats, jackets and blankets.