First and foremost, protect your son against any criminal charges. If none are filed, you can file for a motion to return his property. Better safe than sorry on this one though - I'd suggest at least consulting with a local criminal defense attorney who can assist - and more importantly make sure your son is not still under investigation.
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Sounds like it was a consentual search based on the information you provided. If the computer is evidence in a criminal investigation, they may have limitedrights to seize and preserve the evidence. You need to contact Sac Sherriff High Tech division and ask about a release.
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Consult a criminal defense attorney IMMEDIATELY. Based on your facts, I guarantee your son is not a "witness", he is a suspect. In fact, he's probably the prime suspect if not the ONLY suspect. Say NOTHING more to the police or ANYONE ELSE about this without an attorney present. Get your son an attorney NOW.
The response above is not intended as legal advice. This response does not create an attorney-client relationship. Legal questions can only be fully answered through consultation with an attorney to whom you give full and accurate details. Anything you post here is not confidential and is not protected by the attorney-client relationship. It is highly recommended that you seek advice from a criminal defense attorney licensed in your jurisdiction by setting up a confidential meeting.