When Cameron Paul, a Walmart employee in Joplin, Mo., was growing up, he and his father would go storm-chasing. From what seemed to be a safe distance, Mr. Paul's father, a freelance photographer, would shoot pictures of funnel clouds. In southwest Missouri, there were plenty to keep them busy.

"When other people might have fire drills, we'd have tornado drills," Mr. Paul said. Many residents had storm shelters in their yards, and tornado warnings happened so often, "it was almost like crying wolf," he said.

None of that prepared him for Sunday evening, when Mr. Paul, 19 years old, was working as a customer-service manager at a Walmart Supercenter in Joplin. The employees knew a storm was coming and herded customers to the back of the store, as they'd been trained. Mr. Paul and a co-worker stayed in front, hurrying more people inside from the parking lot. Some said they lived in mobile homes and came to Walmart to seek shelter.

Shortly before the tornado hit, one woman left the building because she wanted to buy groceries and was told she couldn't until the storm passed. Mr. Paul doesn't know what happened to her. Some people who waited in their cars in the parking lot didn't survive.

He stayed by the store entrance until the pressure of the approaching tornado sucked the doors off the building. He and his co-worker ran to the back of the store and dove into the electronics department, where about 30 people huddled. Throughout the giant store, which stretches from 15th to 20th Streets along Range Line Road, somewhere between 100 and 200 people were trying to ride out the storm.