Tornadoes ripped through Alabama and killed at least seven people, including five at a high school where students were pinned under debris from a collapsed roof.
Rescue workers dug through piles of rubble at Enterprise High School looking for other victims.
"The number could very well increase as the search effort continues," said state emergency management spokeswoman Yasamie Richardson.
The burst of tornadoes was part of a larger line of thunderstorms and snowstorms that stretched from Minnesota to the Gulf Coast. Authorities blamed a tornado for the death of a seven-year-old girl in Missouri, and twisters also were reported in Kansas.
In the chaotic hours after the storm, reports about the death toll varied widely. At one point, state officials said as many as 18 people were dead. Richardson later said that miscommunication at the scene was to blame.
"Any time you have a disaster of this magnitude, there is confusion at the scene," Richardson said.
The storm struck at the high school around 1.15pm local time on Thursday, and Richardson said some students were still trapped three hours later.
More than 50 people were taken to hospital as the violent storm front crossed the state. One person died elsewhere in Enterprise and one in rural Millers Ferry, where a separate storm wrecked mobile homes, Richardson said.
Officials opened shelters for those whose homes were damaged.
The state sent in about 100 National Guardsmen, along with emergency personnel, lights and generators.
