my son has a couple of friends that are suspects in a robbery/murder case.

if a search warrant was issued on your home, then there is probable cause to believe that he is involved in a conspiracy to have been involved, despite thatr he actually wasnt the one who pulled the heist..Every person involved in the conspiracy can be prosecuted for the underlying crime. Obviously this is just an opinion without knowing more facts.. Your son should retain an attorney immediately and if arrested he should invoke his right to remain silent.