Walker, who was a senior journalism and economics major, was found dead at at 10:39 a.m. Sept. 7 in his 92 Pine Hill Drive residence.
Thomas Clark III, the associate chief medical examiner handling the Walker case, said a combination of cocaine, alcohol and oxycodone, the main ingredient in OxyContin, found in his system caused his death.
But Clark also added that the levels of each drug detected in Walker's system would have been enough to kill Walker.
"He had enough cocaine in him to kill him, but the combination of the (alcohol and oxycodone) could have killed him too," Clark said.
While investigating Walker's death, officers found some white powder and a partial tablet of a pill on the 20-year-old's desk.
Clark said it is unclear which specific drug or combination of drugs caused Walker's death.
"Either one could have contributed to his death," Clark said.
Clark said it can be difficult to pinpoint the exact cause of death in cases involving recreational drug use.
"With so many possible causes of death the medical examiner is not able to settle on one single drug as the absolute cause," he said.