SPENCER, South Dakota (CNN) -- Rescue crews searched through debris Sunday for more victims after a tornado leveled much of this small town late Saturday, killing at least six people and injuring about 150.

"There's damage as far as the eye can see," Perry Groten, a reporter for KELO, said as he described the scene for CNN. "Houses are totally flat, cars tipped over, and even a mobile home wrapped around a treetop."

Spencer, South Dakota
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Reporters are being told unofficially that the death toll could rise, Groten said.

"Apparently they're finding a lot of victims in apartment buildings where there are no basements," he said. Most of Spencer's roughly 400 residents are elderly and retired.

Gov. Bill Janklow activated the National Guard late Saturday and brought in 50 prison inmates to sweep the one-mile-wide town and help locate victims.

KELO reporter Perry Groten describes the aftermath
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"The entire town is destroyed; there are deceased people and a lot of injured people," Janklow told reporters. Janklow also said he would ask for federal disaster aid.

Some reports suggested the tornado was a half-mile wide.

"About six or eight houses are still standing, and even they're damaged," Captain Dean Strand of the Mitchell County fire Department told CNN.

Police cordoned off the town late Saturday. Power and natural gas lines were cut off to minimize the danger to both search personnel and victims, officials said.

As of Sunday morning, tornado victims who wanted to get back inside the town to survey the damage of their homes were not being allowed in, Groten said.

The McCook County Sheriff's department reported three fatalities to CNN. Queen of Peace Hospital, in nearby Mitchell, reported two others.

Spencer is about 45 miles west of Sioux Falls.