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Emergency crews deployed for battle Wednesday with a vicious ice storm that cut off power for hundreds of thousands of people across the Southeast — and worse power failures were on the way.

About 400,000 customers were without power Wednesday evening in Georgia and the Carolinas, utilities reported. That was on top of tens of thousands more who were in the dark as the storm gathered strength earlier in the week across Louisiana, Arkansas, Texas, Mississippi and Alabama.

"This storm is very unpredictable. It's one of the toughest storms we're going to see in our history," North Carolina Gov. Pat McCrory said Wednesday.

Nearly 600 power workers prepared to camp overnight at the Mall of Georgia in Gwinnett County, northeast of Atlanta, where support staff and volunteers showed up to serve breakfast and dinner and to pack box lunches, NBC station WXIA reported.

Atlanta Mayor Kasim Reed warned that "the hardest part of this storm is tonight and tomorrow," as residents get cabin fever and want to go outside.

"Let me be very clear: Last night and today was the opening act of what's getting ready to occur," Reed said, as snow continues to fall and temperatures plunge back below freezing, re-icing everything.

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It's the ice that's the real culprit — coating streets, trees and power lines to a depth of a full inch in parts of all three states.

Having learned their lesson two weeks ago, when snow locked down the city for a day and a half, Atlantans stayed home, leaving usually jam-packed interstates looking like a sci-fi wasteland as an eerie calm settled over desolate streets slick with ice. Highways were deserted as freezing rain and ferocious wind gusts kept drivers at home.

That wasn't the case in Charlotte, North Carolina's biggest city, where cars and trucks slipped, slid and stalled at the junction of Interstate 277 and Independence Boulevard, a major highway through town.

In a scene reminiscent of the parking lot that Atlanta's freeways became last month, vehicles lined up motionless for miles. Hundreds of crashes were reported, and Independence was eventually officially closed.

Similar scenes played out in South Carolina, where a woman was killed Wednesday when her car speeded out of control and collided with a law enforcement vehicle on Interstate 95 in Clarendon County, Gov. Nikki Haley said.

Three other deaths were also blamed on the storm Wednesday, raising the number of confirmed deaths to 10 since the storm swept in through Texas earlier this week.

The Virginia Department of Emergency Management confirmed Wednesday night that a man was killed in a two-vehicle collision in central Virginia.

A 50-year-old man was found dead outside his home Wednesday morning in Butts County, Ga., apparently of hypothermia, Coroner Ralph Wilson told The Atlanta Journal-Constitution. "He was outside most of the night," Wilson said.

And the Whitfield County, Ga., Sheriff's Office said a woman's body was found Wednesday morning inside her unheated home. The woman, whose name and age weren't immediately released, also apparently died of hypothermia, it said.

At least six other deaths were reported Tuesday as the storm gathered power over Texas and Mississippi: four in separate accidents on icy north Texas roads — including a Dallas firefighter who was knocked off an interstate ramp — and two in separate accidents in Mississippi.

While ice may look pretty and unthreatening, it's actually the biggest hazard of the storm, meteorologists said. The National Weather Service called the ice storm a potentially "catastrophic event" and said some areas of the South could be without power "for days and perhaps as long as a week."

More than 3,400 flights into and out of U.S. airports were canceled and 3,500 more were delayed — the vast majority of them at the Atlanta and Charlotte hubs.

Amtrak suspended service Wednesday on 10 trains in Georgia, Florida and the Carolinas "to reduce the exposure of Amtrak passengers, crews and rail equipment to extreme weather conditions."

As it crawls eastward, the same weather system was forecast to dump up to 12 inches of snow on New York City, Philadelphia and Washington, D.C., overnight and into Thursday, snarling travel plans for millions more people.

"There will be disruption," said Kevin Roth, lead meteorologist for The Weather Channel. "The whole region will be in a deep freeze."

But with warmer temperatures expected, especially along the coast, some of the precipitation will simply be rain, and "there shouldn't be as much ice further north," Roth said.

Daniel Arkin and Alastair Jamieson of NBC News contributed to this report.