Soon after a siren warned of a coming tornado on Friday evening, nursing assistant Linda Knapp took an elderly resident at the Monette Manor nursing home in Monette, Ark., to the bathroom. Then the twister struck, ripping the roof off the building and dislodging chunks of the wall.

Ms. Knapp recalled telling the man to grab a bathroom rail and hold on as tight as he could. She leaned over him to shield him from flying debris, and a piece of concrete struck her. A power outage plunged them into darkness, and water from rainfall or a sprinkler system soaked the floor.