Illinois ? A monster truck performing stunts in front of an auto parts store plowed into a crowd of spectators Thursday, injuring at least nine people, officials said.

Two people were in serious condition at Rockford hospitals, officials said.

Six others, including a mother and her four children, were treated and released, said Sharon Emanuelson, a spokeswoman for Kishwaukee Community Hospital in DeKalb. The other person injured was in good condition.

Another person refused medical treatment, said DeKalb Fire Chief Lanny Russell.

The demonstration, part of a monster truck tour sponsored by NAPA Auto Parts, featured the truck driving over and crushing four cars.

After plowing through the crowd about 2 p.m., the truck went through a fence and stopped on railroad tracks.