Police say the woman was in her own car during the filming when an object went through the windshield of her car during a stunt. She has a serious head injury and is being treated at Loyola Medical Center in Maywood.
The Transformers production team has been filming throughout Illinois and Indiana all summer long.
The crew had been filming at a location in East Chicago for the past three days when something went wrong Wednesday night. A mile stretch on Cline Avenue had been blocked off for production and that's where the accident happened about 6:30 p.m.
"There's a K-rail, the center divider rail that runs the stretch of that road, she was along the K-rail for about half a mile, just grinding on the side of the car," said Blaine Baker.
Baker says he was hired as an extra for the production and Wednesday was his first day on the set. He said the crew did camera run-throughs, but he said he was not sure if there was a rehearsal before the action scene was being filmed.
"A vehicle is being towed by another vehicle, the cable between the two vehicles broke and whipped around and sliced through the woman's car and sliced through her skull apparently," said Baker.
Another witness told ABC7 the cars where moving at a high rate of speed just before the accident.
A medical helicopter was called in to rush the injured extra to Loyola Medical Center.
Indiana State Police is investigating the accident.
"They completely shut it down and the equipment pertaining to the stunt was impounded by the Indiana State Police," said Baker.
It's not clear when filming will resume.
ABC7 was unable to get a comment from the movie studio.