Scott Dixon led the 2009 Indianapolis 500 as the race reached the halfway point under caution following a heavy accident for Tony Kanaan, who crashed out of third position.
Something appeared to break on the right-rear of the Andretti Green Racing machine, but Kanaan emerged from the wreck apparently uninjured.
"I don't know [what went wrong], something broke in middle of straight," said Kanaan. "I hit head on in the first hit, then went for a wild ride. I just rested my head on the rest closed me eyes, I knew it was going to be a big one. I'm pretty beat up.
"The medical team did a good job because I just got beat up pretty big time, and I'm thankful for the safety of the car because that was 190mph and I'm here talking to you. So on to Milwaukee now. Me and this place...."
Dixon, the defending IndyCar Series champion, and last year's winner, took the lead from Penske's Ryan Briscoe on lap 63, as the race went to green following Graham Rahal's accident.
As Briscoe headed for the pits complaining of a lack of grip, Dixon established a small lead over teammate Dario Franchitti.
He briefly lost the lead to Ganassi team-mate Dario Franchitti in the pits during a fourth caution when Davey Hamilton crashed out, but swept straight back by when the race went green on lap 90.
Penske driver Helio Castroneves is currently third, rookie Raphael Matos fourth, Will Power fifth and Danica Patrick sixth.
Marco Andretti has rejoined the race after his lap one accident, 60 laps behind the leader.