NUERBURG, Germany — Race organizers say two-time champion Wolf Silvester has died because of "health problems" during the German VLN Endurance Racing Championship.

Safety marshalls at Saturday's race said Silvester apparently lost control of his Opel Astra OPC, and when they approached the stopped car on the track they found him sitting motionless in the seat.

Attempts were made to resuscitate the 55-year-old driver on the way to the circuit's medical center "where the chief medical officer finally had to testify the death due to an internal medical background."

The dpa news agency reports the driver suffered a heart attack.

Silvester won the 10-race endurance series title in 2006 and 2010.

VLN Chairman Rolf Krimpmann said, "We are deeply concerned and our thoughts are with his family."

The third-round race of the series was canceled.