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Rescuers scoured rubble and expected to find more bodies Monday after the most destructive tornado outbreak of the year killed at least 16 people in three states and hammered the suburbs of Little Rock, Ark.

As day broke over a grim scene of splintered trees and shattered homes there, forecasters warned that tens of millions of people — from Iowa to the Gulf Coast to the Carolinas — were still in the path of a days-long storm system.

“The worst is not over,” said Kevin Roth, a meteorologist for The Weather Channel.

Faulkner County, Ark., particularly the suburbs of Mayflower and Vilonia, was the hardest hit Sunday. As many as 10 people died there, including two children, and authorities said that at least 150 homes had been destroyed. Rescuers still hoped to find people alive.

“What I’m seeing is something that I cannot describe in words,” Faulkner County Sheriff Andy Shock said. “It is utter and sheer devastation.”

In all, 14 people were killed in Arkansas, state emergency management officials said. One woman was killed when debris took out the door of the fortified safe room where she was taking shelter. The state toll was lowered by two at mid-morning, apparently because some victims had been counted twice.

Vilonia, a town of about 4,000 people, was hit by a twister three years ago this week that killed four people and leveled homes and businesses. Homes rebuilt after the 2011 tornado were among those demolished on Sunday night.

On Monday, bulldozers and backhoes were being deployed to help rescuers comb the rubble and look for survivors. Crews tried to be optimistic, but they expected the death toll to climb in the daylight, the sheriff said.

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“We’re praying not, but there’s no telling,” he said. “There’s no knocking on doors. These places are leveled.”

The damage in Arkansas may have been wrought by more than one tornado, spawned from a so-called supercell thunderstorm.

In all, at least 31 tornadoes were recorded. They formed late in the day, and were especially destructive in Arkansas because they struck as night fell. Oklahoma and Iowa reported one death each.

An Arkansas state police spokesman said that 30 cars and trucks were on the road as a twister roared over Interstate 40, and troops went car by car to check on drivers, but it appeared that no one had been killed.

The National Weather Service said that it expected the storms in Arkansas to be rated the strongest in the country this year, perhaps as strong as EF-3 on the weather service’s tornado scale. EF-3 twisters pack wind of at least 136 mph.

In Mayflower, Mark Ausbrooks told The Associated Press that he was at his parents’ house when the storm arrived. He retrieved pillows to put over their heads. The house was destroyed.

“It turned pitch black,” he said. “All hell broke loose.”

At least six shelters were set up in Arkansas to handle dazed survivors. One power company reported that at least 20,000 customers were still without power Monday morning.

It's been a truly awful night for many families, neighborhoods and communities, but Arkansans always step up to help each other recover. -Governor Mike Beebe (@GovBeebeMedia) April 28, 2014

From a trip to the Philippines, President Barack Obama offered condolences and pledged the help of the federal government. He said: “The country will be there to help you recover and rebuild.”

The storms came on the third anniversary of a four-day tornado outbreak that killed more than 300 people across the South.

Tornado season in the United States had been fairly quiet before the storms roared on Sunday. The only death had been a North Carolina infant who was hurt by a tornado on Friday and died at the hospital on Sunday.

For Monday, the threat zone for severe thunderstorms and possible tornadoes covered a swath of the Ohio Valley, the Great Plains and the South, including the cities of Cincinnati, St. Louis, New Orleans, Atlanta and Nashville, Tenn.

The area of greatest concern for tornadoes covered parts of Texas, Louisiana, Mississippi, Alabama, Tennessee and, once again, Arkansas. Little Rock was among the cities covered by a tornado watch until 1 p.m. ET.

#Tornado watch until 12p CT for parts of MS, southeast AR, northern LA, east TX. pic.twitter.com/87VHeOg9zj -TWC Breaking (@TWCBreaking) April 28, 2014

As the storm system churns east, the threat of severe weather is expected to persist through Wednesday.

Using early predictions, forecasters said that the area of highest concern for Tuesday stretched from Indiana south to the Florida Panhandle and east to Raleigh, N.C. For Wednesday, the Virginia coast, the Carolinas and Georgia are in the most danger.

NBC News meteorologist Bill Karins and The Associated Press contributed to this report.