A Petaluma teen who was found dead Jan. 4 in South Lake Tahoe likely succumbed to hypothermia after she ingested large amounts of methamphetamine and other illegal drugs and wandered away from a music festival, authorities said Monday.

Alyssa Byrne, 19, disappeared after she attended a New Year's Eve concert at Lake Tahoe Community College. A utility worker discovered her body three days later about a half-mile away near a snowbank.

The El Dorado County Sheriff's Office released results of an autopsy Monday revealing that the Casa Grande High School grad probably froze to death.

The amount of methamphetamine found in Byrne's system was enough to be considered toxic and also contributed to her death, sheriff's Lt. Pete Van Arnum said Monday.

He said the drug "could have possibly killed her if she hadn't frozen to death first."

Kevin Byrne said Monday that he was "shocked" to learn his daughter had taken methamphetamine.

He said Alyssa had undergone a comprehensive drug screening in November after the teen revealed to her parents that she had been taking Adderall without a prescription. The drug, which contains a combination of amphetamine and dextroamphetamine, is commonly used to treat attention-deficit disorder.

He said no other drugs were found in her system at that time. Nor did doctors believe she needed Adderall to help her focus.

The results of the autopsy showed no evidence of alcohol in the teen's system at the time of her death.

However, she had ingested "multiple psychoactive drugs" in addition to the methamphetamine. Van Arnum did not identify those substances, which he said are illegal, at the request of Byrne's family.

Kevin Byrne said he didn't ask what the other drugs were when he spoke with authorities on Sunday. Nor does he recall asking that the information be withheld from the public.

He said he and his wife, Kim, have not gotten the clothes their daughter wore that night or the cellphone she used to keep in touch with friends.

Van Arnum said Nevada authorities are still investigating where Byrne obtained the drugs, including whether she did so in Tahoe or somewhere else.

She was with about 40,000 people who attended the SnowGlobe Music Festival, which Van Arnum said "draws a lot of people who like to party."

Kevin Byrne said he believes his daughter wasn't aware of what she was putting into her body the night she died.

"I've talked to all the kids and told them I wish they would have called me and told me this is what Alyssa is doing. I would have been the first one to drive up to Tahoe."

Byrne was attending Santa Rosa Junior College in hopes of becoming a firefighter/paramedic. Authorities believe she may have elected, like many other concert-goers, to avoid long shuttle-bus lines and walk back to the Stateline, Nev., hotel area where Byrne was sharing a room with three friends at the Horizon Casino Resort.

Using funds donated in memory of their daughter, the Byrnes and others have launched a public service campaign promoting a buddy system to help keep teens and young people out of danger.

Kevin Byrne said his daughter's autopsy results only strengthen his desire to promote that message.

"If someone knows that their buddy is using drugs, it's up to them to contact a friend or a parent," he said.

You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.

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