Ferrari's Formula One championship leader Fernando Alonso and McLaren's Lewis Hamilton crashed out of the Belgian Grand Prix at the start on Sunday.
The safety car was deployed immediately after the collision into the first corner that also led to the retirements of Sauber's Mexican Sergio Perez and Lotus's French driver Romain Grosjean.
Television replays indicated that Grosjean, whose car flew through the air over the nose of the Ferrari which also lifted off the ground, had triggered the crash by cutting across into Hamilton who had nowhere to go.
Debris from the cars littered the track.
Alonso had been set to equal Michael Schumacher's Formula One record of 24 successive points scoring finishes and has a 40 points lead over Red Bull's Mark Webber in the championship with eight races remaining after Spa.
The Spaniard, who could consider himself fortunate not to have been struck on the head by the Lotus, was slow to get out of the car but seemed unhurt.
Japan's Kamui Kobayashi, who had started on the front row for Sauber, pitted after the accident and resumed in last place with McLaren's Jenson Button leading.
Stewards said they would investigate the turn one accident after the race.
They were also investigating what looked like a jumped start by Venezuelan Pastor Maldonado's Williams from sixth place. Maldonado had already been penalised three places on the grid for impeding in Saturday's qualifying.
Photograph: Ker Robertson/Getty Images