Silliman's body was discovered in an empty home in the New Hill area of Wake County in November 2008. According to the medical examiner, he was drugged, strangled and suffocated.
Detectives have accused four former friends in his death: Aadil Shaaid Kahn, Allegra Rose Dahlquist, Drew Logan Shaw, and Ryan Patrick Hare.
All of the defendants, except Hare, were under 18 at the time of the crime and the former three have agreed to testify against him in a murder trial set for May.
Now, investigators allege Hare, Kahn, and Dahlquist tried to kill Silliman in a separate incident five days before his death.
The three now face additional charges of attempted first-degree murder and conspiracy to commit first-degree murder.
Shaw allegedly did not take part in the earlier incident and does not face additional charges.