Authorities in this southwest Missouri city hammered by a tornado that killed at least 122 people believe that many, if not most, of those considered "unaccounted for" survived the storm safely.

Still, they sent rescue teams back out Wednesday to comb through the debris for a fourth time in a search for survivors of the nation's deadliest single tornado since 1950.

Authorities said it is impossible to know exactly how many people are missing and have cautioned the fact that people are unaccounted for does not necessarily mean they are still trapped in debris or have died.

"There has been information in news sources that 1,500 people are missing," City Manager Mark Rohr said. "That does not mean they are injured or deceased. It means that loved ones are not aware of their whereabouts. We understand that some people may have been out of the area when the storm hit or have since left."

Rohr and Fire Chief Mitch Randles declined to speculate on whether the death toll from Sunday's storm will grow substantially. It wasn't clear Wednesday when the search operation would switch from rescue to recovery.

More On This...

Randles said rescuers are confident that no additional bodies will be found at one of the hardest hit areas in Joplin, a Home Depot store that was flattened by the tornado.

Bob Benson, a Red Cross official who set up a "Safe and Well" registry at a shelter on the Missouri Southern State University campus, said a small number of parents have arrived looking for lost children. More than 100 people have come to him seeking information on missing senior citizens.

Social networks were the tool of choice for many people trying to track the missing -- or to let their loved ones know they were OK.

Several online efforts have focused on Will Norton, a teenager who vanished on his way home from his high school graduation ceremony. Norton was driving with his father, Mark Norton, when the storm hit his Hummer H3. The vehicle flipped several times, and Will was thrown from it, likely through the sunroof.

Sara Norton was on the phone with her father as the two drove home. Mark Norton asked her to open the family's garage door so Mark and Will could get inside quickly. But the two never made it.

I could hear him saying, `Will, pull over, pull over,"' Sara Norton said.

Mark Norton tried to grab his son, but the storm was too strong. He was hospitalized Tuesday, seriously hurt but still able to talk to his family about what happened.

Will's sister, Sara Norton, and other relatives drove to hospitals throughout Missouri to search for Will. More than 19,000 people supported the "Help Find Will Norton" community page on Facebook, and Twitter users were tweeting heavily about the missing teen.

"I just want to find him, that's all," Sara Norton said Tuesday, on her way home from a Springfield, Mo., hospital. "I'm just determined. I have to find him."

Many posted prayers for Norton's safe return or repeated rumors about where he might have been taken. Others commented on videos that Norton, an avid videographer with plans to study film in college, had posted on YouTube.

Joplin schools were ravaged by the twister and classes have been canceled the rest of the school year, but district officials are trying to locate both faculty and many of the school's 2,200 students. The effort has been crippled by downed phone lines. Some students have been located using Facebook.

"We just want to be able to find who we can find and then as confirmation happens offer support to the families if we find out that a kid didn't make it," Joplin High Principal Kerry Sachetta said. "When a tragedy happens for a kid or a family, everybody tries to come together and console everybody and make up what we can whether it is food or emotional support or a place to stay. That's what we are trying to do a little piece at a time."

Bill Davis, the lead forecaster on a National Weather Service survey team, said he would need to look at video to try to confirm that. But he said the strength of the tornado was evident from the many stout buildings that were flattened: St. John's Regional Medical Center, Franklin Technology Center, a bank gone except for its vault, a Pepsi bottling plant and "numerous, and I underscore numerous, well-built residential homes that were basically leveled."

Davis' first thought on arriving in town to do the survey, he said, was: "Where do you start?"