JOPLIN, Missouri - 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.

Members of the St. Louis Metro Urban Search & Rescue Strike Team 3 conduct a secondary search for victims of a tornado after it struck Joplin, Missouri, May 24, 2011. [Photo/Agencies]

Across this devastated city, searchers moved from one enormous 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 that killed at least six people.

Nine survivors had been pulled from the aftermath in Missouri, and searchers fought the clock because anybody still alive after the deadliest single tornado in 60 years was losing precious 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 both hospitalized after the twister hit their house, would be found.

She showed up 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. Burns was weary but composed. Her daughter -- the boy's aunt -- sobbed next to her.

"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 on Sunday was an EF-5, the strongest rating assigned to tornadoes, with winds of more than 200 mph (320 kph). 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 US history.

Authorities said an estimated 750 people were hurt and an unknown number were still unaccounted for, but authorities were reluctant to declare a specific number.

"That does not mean they are injured or deceased. It means that loved ones are not aware of their whereabouts," City Manager Mark Rohr said, explaining that many had probably left the area without telling family members.

Another top job was testing the city's tornado sirens to make sure they were operable ahead of another round of potentially violent weather starting Tuesday evening and expected to last into Wednesday in some places. Emergency officials warned jittery residents well in advance of the test.

David Imy, a meteorologist at the federal government's Storm Prediction Center in Norman, Oklahoma, said conditions were ripe for severe thunderstorms, including tornadoes, in parts of Kansas, Missouri, Arkansas, Texas, Kentucky and Tennessee, as well as nearly all of Oklahoma.

Throughout the search efforts, new reports emerged of clusters of victims: 11 people dead in a nursing home, three bodies found in an Elks Lodge.

The tornado tossed three vehicles into the Greenbriar nursing home and left nothing more than a 10-foot (three-meter) section of an interior wall standing.

Jasper County Emergency Director Keith Stammer said the scope of the destruction was making it difficult to account for people affected by the storm. He suggested that many survivors, with nowhere to go, left Joplin for Kansas, Arkansas, Oklahoma or other parts of Missouri.

"There's a lot of confusion, a lot of inability for folks to communicate," he said.

Authorities also announced a curfew from 9 pm to 6 am, with only residents and emergency workers allowed inside the disaster zone.

People in the Joplin area and beyond have turned to online social networks to find family members missing since the tornado or to learn about the plight of survivors.

Multiple Facebook pages created since the tornado are filled with requests for information about specific people who have not been heard from since Sunday. Some pages include photos of the missing. Other posts share the news about Joplin residents who are alive and well.

Several social-networking efforts specifically focused on finding information about Will Norton, a teenager who vanished on his way home from his high school graduation ceremony. More than 10,000 people have supported the "Help Find Will Norton" community page on Facebook, and Twitter users were tweeting heavily about the missing teen.

Family members told The Associated Press that Norton and his father were still on the road when the storm hit. Mark Norton urged his son to pull over, but the teen's Hummer H3 flipped several times, throwing the young man from the vehicle, likely through the sunroof.

The elder Norton was hospitalized.

From the air, the difficulty of the search was apparent.

The tornado damage was "like taking a mower through tall grass. That's what it looks like," said state Sen. Ron Richard of Joplin, who flew over the area with Missouri Gov. Jay Nixon and US Sen. Claire McCaskill. He described the devastation as "down to the ground."

The Home Depot was identifiable only by the prevalence of the store's signature orange color in the corrugated roofing and metal framing that looked almost as if it had been melted.

Jackhammers pounded against heavy concrete slabs that once held up the store. Crews were desperate to punch through so dogs could sniff for any scent of people below. A day earlier, rescuers found one person alive in the store's wreckage but also recovered seven bodies under the concrete.