Joseph Loudon, 16, died on May 23 after paramedics found him unresponsive at a home in the 100 block of Hillcrest Drive in Orinda.
A large party had been held at the house, sheriff's spokesman Jimmy Lee said.
Loudon was taken to Kaiser Permanente Walnut Creek Medical Center, where he was pronounced dead.
Toxicology tests found alcohol and the pharmaceutical drug papaverine in Loudon's system, although his blood-alcohol level was very low, Lee said.
Orinda police later arrested 18-year-old Patrick Gabriella on suspicion of contributing to the delinquency of a minor and furnishing alcohol to a person under 21 years old.
A 16-year-old boy was also arrested for allegedly furnishing alcohol to a minor.
During the investigation, police discovered that several students at Miramonte High School, which Loudon attended, had fake ID cards.
Investigators were told that 22-year-old Rohnert Park resident Robert Scott had provided the teenagers with the forged driver's licenses.
When police went to arrest Scott, they allegedly found about 100 fake licenses in his home, police said.
Scott was booked into the Martinez Detention Facility on suspicion of forgery and identity theft, both felonies, according to police.
**Please Note: Since this report was first published, ABC7 has learned officials were incorrect in reporting Loudon died with papaverine in his system. It was learned on September 14, 2009 that an organ donation staff member injected the drug into Loudon's body after he died. More on that story here: Orinda teen didn't die with drugs in system