The Apex teen was found murdered in an empty home in the New Hill area of Wake County in December.
Detectives have accused four former friends in his death. Aadil Shaaid Kahn, Allegra Rose Dahlquist, Drew Logan Shaw, and Ryan Patrick Hare could face life in prison if convicted.
Now, a newly released search warrant in the case says detectives interviewed another Apex teen in their investigation. The warrant says the four suspects had been talking with Silliman's girlfriend Michelle Lippert on the social website Facebook.
In the warrant, detectives say they searched her home for a video of Silliman taken by one of the four suspects in the hours before he was last seen by his parents.
The warrant also says "The Facebook.com chats and interview of Ms. Lippert confirm that Ms. Lippert had knowledge of Mathew Silliman's disappearance and did not report this knowledge to the Authorities when a Silver Alert was issued for Mathew Silliman."
Investigators took cameras, computers, and data disks from the Lippert home.
To date, Lippert has not been charged with any crime.