Aadil Shaaid Kahn, 18, entered his plea Wednesday. Like his co-defendant, 19-year-old Allegra Rose Dahlquist, Kahn pleaded to two counts of conspiracy, attempted murder and second-degree murder.
A judge will sentence Kahn after he testifies at 19-year-old Ryan Patrick Hare's trial. Jury selection begins on September 7, and the trial is expected to begin on September 13.
Earlier this month, Dahlquist entered a guilty plea to the lesser charge of second-degree murder and agreed to testify against Hare.
Prosecutors say Hare developed a plan to kill 18-year-old Silliman and got Dahlquist and Kahn to help out.
A fourth defendant, 18-year-old Drew Logan Shaw, is cooperating with the investigation and also will testify against Hare, according to the district attorney's office. The ADA, Shaw will make a plea agreement.
Shaw's case will be settled after the completion of the Hare trial.
The teens were arrested in December 2008 following the discovery of Silliman's body in a vacant home in New Hill -- less than two weeks after Silliman's parents reported him missing on November 26.