Extra Gabriela Cedillo was injured on the set Wednesday night along a stretch of Cline Avenue, which was closed to traffic for filming.
A stunt was apparently being performed during filming in East Chicago, Indiana, when Cedillo, who is not a stuntman, was hurt.
"The vehicle was being towed by another vehicle," said fellow extra Blaine Baker. "The cable between the two vehicles broke. It whipped around and sliced through the woman's car and sliced through her skull, apparently."
Baker said the car was then dragged along a center divider for about half a mile, "grinding the side of the car off."
A medical helicopter was called in to airlift the injured driver to Loyola Medical Center. Indiana State Police are now investigating the accident.
"They completely shut it down. The equipment pertaining to the stunt was impounded by the Indiana State Police," said Baker.
Filming scheduled to take place Thursday at 6 a.m. in Millennium Park has been canceled. Thursday was set to be the last day of "Transformers 3" filming in Chicagoland. It's unknown whether Thursday's cancellation is actually due to the accident. Security personnel at Millennium Park say other shoots have been canceled last-minute as well.