Whatcom County Medical Examiner Dr. Gary Goldfogel says Whatcom Middle School student Hazel Phair had alcohol poisoning when she came home after a night of drinking early September 29.
Phair also had marijuana in her system.
Her mother checked on her a few hours later and found her unresponsive.
Police had originally thought Phair suffocated on her own vomit after falling asleep.
Authorities say there will be charges for anyone involved in supplying Phair with the alcohol and drugs.