In addition, seven people found in debris were rescued early Monday, KSPR-TV, Springfield, Mo., reported. Rescue efforts have been hampered by rain and high winds, the TV station said.
The National Oceanic and Atmospheric Administration said Monday the tornado was the deadliest single tornado in the United States since 1953.
Preliminary estimates indicate the tornado was rated as an EF-4, the second-highest classification, but it may eventually be rated EF-5, KSPR-TV said. An EF-4 tornado features winds of 166 mph to 200 mph.
Nixon toured the devastated area one day after the tornado cut a 6-mile swath, uprooting trees, destroying buildings, sparking fires and tossing vehicles around like toys.
"This tornado basically started over Joplin and stayed there for a long time," Nixon told The New York Times. "It is devastating but we are working hard to continue to find those that are still alive."
Rescue workers checking wrecked buildings found five families alive, Nixon said. Scores of people remain missing and at least 89 were killed, he said.
Rescue efforts were complicated by storms battering the area Monday with hail and heavy downpours, Accuweather.com reported.
The Joplin tornado was one of 50 reported Sunday night in Missouri, Minnesota and Wisconsin. Another tornado outbreak was predicted for the South Tuesday, Accuweather.com said.
Joplin City Manager Mark Rohr said the Sunday night tornado, described by one resident as "quite horrific," sliced through the center of the city.
The Joplin tornado struck the heart of the city "and it's very dense in terms of population," Emergency Management Director Keith Stammer told CNN. He said more than 40 rescue units from Kansas, Arkansas, Oklahoma and Missouri were sent to help.
The Springfield (Mo.) News-Leader said St. John's Regional Medical Center in Joplin took a direct hit. KSHB-TV, Kansas City, Mo., reported fires throughout the hospital.
Nearly 200 patients were evacuated. Many were taken to Freeman hospital in Joplin and others were flown to other St. John's hospitals in Springfield and northwest Arkansas, KSPR-TV reported.
X-rays from St. John's were found in driveways 70 miles away, said Ray Foreman, a meteorologist with KODE-TV, Joplin.
Nixon declared a state of emergency and activated the National Guard to help deal with the disaster.
U.S. President Barack Obama, who is visiting Ireland, called Nixon and pledged aid from the Federal Emergency Management Administration, the White House said.
"The president has directed FEMA Administrator Craig Fugate to travel to Missouri to ensure the state has all the support it needs," a spokesman said, and a FEMA team is en route to Joplin.
Joplin High School, a Home Depot, a Walmart, a church, apartments and gas stations were among buildings heavily damaged or destroyed. Schools were closed Monday.
Jerry Williams, assistant vice president at Missouri Southern State University, told the News-Leader he heard the tornado coming.
"I took my wife into a closet under the stairs," he said. "It sounded like a huge wind.
"It's been quite horrific. There are just areas that are flattened. Places are gone."
Jeff Law, 23, rode out the tornado in a friend's storm cellar. When he came out afterward, he said he didn't recognize his town.
"I didn't know where I was," Law said. "Everything was unrecognizable. It's like Armageddon."
In Minneapolis, police declared an exclusion zone Monday in part of the tornado-pummeled North Side, where only residents will be allowed in once the area is considered safe, the Star Tribune newspaper in Minneapolis reported.
One person was killed and 29 hurt in at least three tornadoes that struck the Twin Cities Sunday. WCCO, Minneapolis, reported 30 homes were damaged.
Hundreds of people were forced to seek shelter from their damaged homes, schools were closed and about 10,000 customers were without power Monday.
Although rumors of looting proved largely unfounded, a 9 p.m. to 6 a.m. curfew imposed Sunday will continue for the next "couple of days," Minneapolis Mayor R.T. Rybak said.
The National Weather Service said twisters were reported in Juneau, Taylor, La Crosse and Monroe counties in western Wisconsin, the Milwaukee Journal Sentinel reported. There were no immediate reports of injuries.
The windstorms knocked down trees in Mather and flipped several cars, damaged billboards and broke windows in Sparta, the newspaper said. In La Crosse, a tornado blew roofs off buildings and caused serious damage to a Kmart store. Authorities were checking a report of people possibly trapped in an apartment building, the Journal Sentinel said.
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