Three adults and six children were taken to Kishwaukee Community Hospital in DeKalb, officials said.
Two of them, a mother and her 4-year-old daughter, were transferred in serious condition to trauma hospitals in Rockford, said Kishwaukee hospital spokeswoman Sharon Emanuelson. The seven others were treated and released by Thursday night.
The demonstration, part of a monster truck tour sponsored by NAPA Auto Parts, featured the truck driving over four cars, crushing them.
"After its third or fourth attempt of rolling over the cars, and getting back down to the street, it appeared to have lost control and at that point careened off to the left into part of the crowd that was watching the event," said City Manager Mark Biernacki.
Witness Patrick Sheridan, 16, said the driver and staff from NAPA had asked observers to stand back from the road after the truck's first few passes.
"I watched the guy go over two or three times getting some air and one time he went up and he came really close to hitting people," Sheridan said, adding that the truck appeared to come within just three feet of bystanders.
The teen said the final pass "was like a full-out jump because he, like, floored it and it just went up and landed on the back tires and kept going."
"It sounded like he couldn't get the gas off and like it kept going and going and going," he said.
Biernacki said the 44-year-old truck driver did not appear to be injured in the accident that happened about 2 p.m.
"There was just this sound of steel crunching," DeKalb resident James Vesely told The Daily Chronicle of DeKalb. "I ran over to see if it hit anybody's car, and I saw a woman and a little girl ... lying on the ground with dirt on them."
One of the two seriously injured victims was transferred to OSF St. Anthony Medical Center and the other was sent to Rockford Memorial Hospital, said Kishwaukee nursing supervisor Jennifer Cedillo. OSF Saint Anthony spokeswoman Therese Michels confirmed that one of the victims was at the hospital's trauma center, but Rockford Memorial declined to release any information Thursday night.
Jerry Nix, a spokesman for NAPA's parent company, Genuine Parts Co., said he could not comment on the incident. A message left at the auto parts store near the accident site after business hours was not immediately returned.
The city of DeKalb gave the store permission to close the street for the demonstration, Russell said. DeKalb police had no further details Thursday.
The manager of a bicycle shop on the block where the accident happened said he didn't see the incident but saw the truck performing stunts beforehand.
"It looked very precarious," said Tobie DePauw, manager of North Central Cyclery.