Michael Schumacher suspects a mechanical failure caused his collision with Jean-Eric Vergne during the Singapore Grand Prix.
Schumacher's Mercedes slammed into the back of Vergne's Toro Rosso as the Frenchman battled with Sergio Perez's Sauber on the restart lap after the race's first safety car period.
"After the safety car I braked a little bit earlier than I would normally do for that corner," Schumacher said. "The car wouldn't decelerate and I need to investigate why that was the case.
"I just went straight on the brakes very hard, locked up everything and tried to brake as hard as possible but the accident wasn't avoidable any more."
Schumacher expressed his sympathy for Vergne, who had looked set for a points finish.
"Obviously I feel sorry for Jean-Eric that he had to retire and for the guys in my team, but we have to find out what happened," said the seven-time champion.