Raleigh, N.C. — Apex teen Matthew Silliman had lethal amounts of prescription drugs and alcohol in his system when he suffocated, a toxicology report released Tuesday shows. Silliman died of asphyxiation, an autopsy report released by the Office of the Chief Medical Examiner concludes.

Wake County deputies found the 18-year-old's body on Dec. 2 in a mobile home at 4221 Olive Branch Lane in New Hill after receiving a tip from Apex police.

The autopsy report says Silliman's body was found zipped into a sleeping bag. He was bound hand and foot with plastic ties, and his mouth was covered with duct tape. A plastic bag covered his head. It, too, was secured with plastic ties.

Samples taken from Silliman showed a "concentration of bupropion ... potentially lethal ... along with acute ethanol intoxication" that likely sedated the teen before he suffocated, according to the autopsy.

Bupropion is most commonly used as an anti-depressant. It is an ingredient in the drug Wellbutrin.

Silliman disappeared Nov. 26 and was initially the subject of a Silver Alert – a statewide system that notifies the public about missing endangered adults who suffer from a mental cognitive impairment. At the time, Apex police said Silliman suffered from depression and bi-polar disorder.

According to arrest warrants, he was killed Nov. 30.

Four of Silliman's acquaintances – Allegra Rose Dahlquist, 17, of 601 Walcott Way, Cary; Ryan Patrick Hare, 18; of 100 Walnut Hill Court, Apex; Aadil Shahid Khan, 17, of 901 Bristol Blue St., Apex; and Drew Logan Shaw, 16, of 107 Woolard Way, Apex – were indicted on one count of murder each in the case.

Hare, Shaw and Dahlquist were also indicted on one count each of conspiracy to commit murder.

According to search warrants in the case, one of the accused confessed to being involved in Silliman's death and implicated three others. Of the other three, two more confessed to their involvement. The three who confessed implicated the fourth suspect as being directly involved in his death.

One of the suspects admitted to beating Silliman on the head with a hammer, tying him with zip ties, suffocating him with duct tape and disposing of him in a plastic bag, the warrants stated. The suspect also told investigators they tried to use medication commonly used for animals to plan the crime.

The assailants also read Silliman's fortune to him off tarot cards before his death, the warrants stated.

Wake County real estate records say the property where Silliman was found belongs to Dahlquist's family.

Authorities have said little about the case, but Silliman's page on Facebook, an online social networking site, lists the suspects as his friends.

Dahlquist and Khan were seniors at Apex High School, a Wake County schools spokesman said. Shaw was a sophomore at Panther Creek High School in Cary. Hare withdrew from Panther Creek in February.