The Harrisburg tornado was an EF4, the second-highest rating given to twisters based on damage. Scientists said it was 200 yards wide with winds up to 170 mph.
Adding to the danger was the storm's timing: It hit when many people were fast slept.
Meteorologist Harold Brooks called that unusual but "not completely uncommon."
Brooks, with the National Severe Storms Laboratory in Norman, Okla., said perhaps 10 percent of tornadoes strike between midnight and 6 a.m., a time when storms are harder to spot, and it's harder to get the word out.
"If you're asleep, you're less likely going to hear anything, any warning message on the danger," Brooks said.
That didn't appear to be the case in Harrisburg, where the mayor credited storm spotters for saving lives.
In eastern Tennessee, where three people were killed, donated storage units were to be offered to families whose homes were damaged so they could store possessions before the next line of storms.
The brother-in-law of a Tennessee woman who was killed said he found her under some debris and held her hand until paramedics arrived.
George Jones and several relatives gathered Thursday at the shattered mobile home of Melissa Evans Beaty outside the small city of Crossville, about 110 miles east of Nashville.
He said Beaty, whom they called Lisa, was alive and asking about her grandchildren when he found her.
The children were all right, but Beaty's husband, Ricky, was taken to a Knoxville hospital with a fractured pelvis and severe head trauma, Jones said.
"We would give anything, to have Lisa back," he added.
The couple's home was completely destroyed, with pieces wrapped around nearby trees.
Bunny Howe survived the storm with her 9-year-old grandsons by climbing into a bathtub as she watched the wind pick up one of her horses in the back yard, then overturn part of a tractor-trailer in the front yard.
That's when she got on top of the children and held the bathroom door shut with her feet.
The children asked her, "Ma, what are we going to do?'" She told them, "We're going to pray."
The tornado tore off a wall of the Howes' garage and toppled a tree onto the roof.
After all that, Howe said, she's not overly concerned about storms predicted for Friday.
"What's it going to do?" she asked. "Take the rest of the house down?"
Associated Press Writer Kristin M. Hall in Crossville, Tenn., contributed to this report.
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