One of the drivers still trying to qualify for the Indianapolis 500, Stephan Gregoire, spun into the wall during practice.
He was examined at the infield hospital and transferred to Methodist Hospital for observation after complaining of pain in his back.
Gregoire, whose top speed of the day was 217.851 mph (350.58 kph), had just completed a lap at almost 215 mph (346 kph) when his Panoz race car got sideways and backed into the wall coming out of the first turn.
The 38-year-old Frenchman has been in seven races at Indianapolis, with his best finish eighth in 2000.
It was the fourth crash since practice began May 6. Jon Herb, Milka Duno and Jimmy Kite were not injured in the earlier incidents.