Marco Simoncelli died as a result of massive trauma to his head, neck and chest, MotoGP's medical director has confirmed.
Claudio Macchiagodena said the Gresini Honda rider was unconscious when the medical crew reached him on the circuit.
They performed CPR in both the ambulance and for 45 minutes in the medical centre, but were unable to save his life.
"Because of the bad crash to him during the race, he was hit by other riders, he was reported with a very important trauma to the head, to the neck, and to the chest," Macchiagodena reported.
"When our medical staff arrived on him he was unconscious. In the ambulance, they started CPR [cardiac pulmonary resuscitation].
"Immediately in the medical centre he was incubated, and it was possible to evacuate and take some blood out from the thorax.
"The CPR was continued for 45 minutes because we thought to help him as long as we thought it was possible.
"Unfortunately it was not possible to help him and at 16.56 we had to declare he was dead."
Race director Paul Butler said the accident would be investigated, but said that was for another occasion.
He said all thoughts were with Simoncelli's family: "Our sympathies go to his family. We want to show every respect possible to him."