The US state of Missouri has released a list of more than 230 people missing since a devastating tornado struck the city of Joplin on Sunday.
But the list is shrinking as people are located, and officials hope others have simply failed to contact relatives.
The tornado killed at least 125 people, injured 750 and wrecked as much as a third of the city, making it one of the most destructive in US history.
US President Barack Obama plans to visit Joplin on Sunday.
On Thursday, the Associated Press found Sally Adams, 75, alive and well.
Neighbours rescued her after Sunday's tornado, but she had lost her mobile phone and was unable to contact her family, who reported her missing to authorities.
Another series of tornadoes struck Oklahoma, Arkansas and Kansas on Tuesday and Wednesday, killing at least 16 people.
On Thursday, Oklahoma authorities discovered the body of a three-year-old boy who went missing after a tornado struck his home.
'We are hopeful'
Search teams in Joplin, Missouri, home to 49,000 people, have scoured the wreckage for survivors. But by Thursday morning, authorities began weighing when to begin using bulldozers and other heavy kit to clear debris - a step that would indicate no more survivors were expected to be found.
Joplin Fire Chief Mitch Randles was quoted by the Associated Press as saying: "We've had stories from earthquakes and tsunamis and other disasters of people being found two or three weeks later, and we are hopeful we'll have a story like that to tell."
Those leading the search effort say it is impossible to know how many people are truly missing, since many may have simply left the area and not been in contact with relatives.
Mike Hare said his 16-year-old son Lantz was among the missing. Mr Hare told AP he had searched the neighbourhood where the boy was last seen and had called hospitals as far away as Dallas and Kansas City.
Lantz Hare had been driving with a friend when the tornado struck on Sunday evening. The tornado destroyed the car, and Mr Hare found Lantz's backpack in the wreckage.
"We know he's hurt somewhere," Mr Hare said on Wednesday. "We just can't sit and keep calling. You've got to be moving."