JOPLIN, Mo. -- Emergency crews drilled through concrete at a ruined Home Depot, making peepholes in the rubble in hopes of finding lost shoppers and employees. A dog clambered through the shattered remains of a house, sniffing for any sign of the woman and infant who lived there.

Across this devastated city, searchers moved from one debris pile to another Tuesday, racing to respond to any report of a possible survivor.

As the death toll in Joplin rose to at least 122, another line of severe thunderstorms spawned tornadoes in Oklahoma and Kansas Tuesday that killed at least six people.

Nine survivors had been pulled from the aftermath in Joplin, and searchers fought the clock because anybody still alive after the deadliest single tornado in 60 years was losing strength two days after the disaster. And another round of storms was closing in.

For Milissa Burns, hope was fading that her 16-month-old grandson, whose parents were hospitalized after the twister hit their house, would be found.

She arrived Tuesday at a demolished dental office near the family's home to watch a search team. At one point, a dog identified possible human remains, prompting eight searchers to dig frantically, but they came away with nothing. "We've already checked out the morgue," Burns said. "I've called 911 a million times. I've done everything I can do. He was so light and little. He could be anywhere."

Also Tuesday, the National Weather Service announced that the twister that crippled Joplin was an EF-5, the strongest rating assigned to tornadoes, with winds of more than 200 mph. Scientists said it appeared to be a rare "multivortex" tornado, with two or more small and intense centers of rotation orbiting the larger funnel.

It was the deadliest single twister since the weather service began keeping official records in 1950, and the eighth-deadliest in U.S. history. Authorities said an estimated 750 people were hurt and an unknown number were still unaccounted for.

A 54-year-old woman died after suffering a heart attack when told of her father's death in the Joplin tornado.

Newton County Coroner Mark Bridges says the woman who died Tuesday didn't immediately learn of her father's death in the historic storm that has killed more than 120 people. He did not know her father's age or the circumstances of the man's death. Bridges identified the woman as a schoolteacher and resident of nearby Webb City.