JOPLIN, Mo. - Rescue crews dug through piles of splintered houses and crushed cars Monday searching for victims of a tornado that blasted much of this Missouri town off the map and left at least 116 people dead.

It was the deadliest single twister in nearly 60 years and the second tornado disaster in less than a month.

As the full scope of Sunday's calamity came into view - houses reduced to slabs, cars crushed like soda cans, shaken residents roaming the streets in search of missing family members - authorities feared the toll would rise.

The danger was by no means over. Fires from gas leaks burned across town, and more violent weather loomed, including the threat of hail, high winds, and even more tornadoes.

At daybreak, the city's south side emerged from darkness as a barren, smoky wasteland.

"I've never seen such devastation - just block upon block upon block of homes just completely gone," said Gary Burton, a former state legislator, who showed up to help at a volunteer center at Missouri Southern State University.

Unlike the multiple storms that killed more than 300 people last month across the South, Joplin was smashed by just one exceptionally powerful tornado.

Not since a June 1953 tornado in Flint, Mich., had a single twister been so deadly. That storm killed 116, according to the National Weather Service.

Authorities were prepared to find more bodies in the rubble throughout this blue-collar town of 50,000 people 160 miles south of Kansas City.

Gov. Jay Nixon, who received a phone call from the traveling President Obama offering condolences and pledging help, said he did not want to guess how high the death toll would eventually climb. But he said: "Clearly, it's on its way up."

By evening, 17 people were pulled alive from the rubble. Local news reports said more than 1,000 had sought treatment at hospitals.

While many residents had up to 17 minutes of warning, rain and hail may have drowned out the sirens.

Larry Bruffy said he heard the first warning but looked out from his garage and saw nothing. "Five minutes later, the second warning went off," he said. "By the time we tried to get under the house, it already went over us."

As rescuers toiled in the debris, a thunderstorm lashed the city. Rescue crews had to move gingerly around downed power lines and chunks of debris as they looked for victims and hoped for survivors. Fires, gas fumes, and unstable buildings posed constant threats.

Teams of searchers fanned out in waves across several square miles. The groups went door to door, making quick checks of property that in many places had been stripped to their foundations or had walls collapse.

National Weather Service Director Jack Hayes said the storm was preliminarily labeled an EF4 - the second-highest rating assigned to twisters based on the damage they cause.

Hayes said the storm had winds of 190 to 198 m.p.h. At times, it was three-quarters of a mile wide.

Some of the most startling damage was at St. John's Regional Medical Center, where staff had only moments to hustle their patients into hallways. Six people died there, five patients and one visitor.

The storm blew out hundreds of windows and caused so much other damage that doctors abandoned the hospital soon after. A crumpled helicopter lay on its side in the parking lot near a twisted mass of metal that used to be cars.

Dr. Jim Riscoe, working at Joplin's Memorial Hall entertainment venue, said some members of his emergency-room staff showed up after the tornado with injuries of their own, but they worked through the night anyway.

"It's a testimony to the human spirit," he said.

Once the center of a thriving mining industry, Joplin flourished though World War II because of its rich lead and zinc mines. It also gained fame as a stop along Route 66, the storied highway stretching from Chicago to Santa Monica, Calif., before freeways diminished the city's importance.

Joplin, named for the founder of the area's first Methodist congregation, now is a transportation crossroads and manufacturing hub. It's the hometown of poet Langston Hughes and Gunsmoke actor Dennis Weaver.

Major employers in and around the city include the electronics manufacturer LaBarge Inc., colleges such as Missouri Southern State, and hospitals and clinics. Agriculture is also important to the economy.

As the tornado bore down on their trailer home, Joshua Wohlford, his pregnant girlfriend, and their two toddlers fled to a Wal-Mart store. The family narrowly escaped after a shelf of toys partially collapsed, forming a makeshift tent that shielded them.

"It was 15 minutes of hell," Wohlford said.

At a Fast Trip convenience store, 20 people ran into a pitch-black cooler as the building began to collapse around them. They documented their experience with a video that was drawing tens of thousands of views online by Monday afternoon.

The audio was even more terrifying than the imagery - earsplitting wind, objects getting smashed, wailing children, and a woman praying. No one was seriously hurt.

The twister that hit Joplin was one of more than 50 reported across seven Midwest states over the weekend. One person was killed in Minneapolis and another in Kansas.