Garry Shandling: Doctor Refuses to Sign Death Certificate ... Coroner Launches Investigation

Garry Shandling has been dead for nearly 48 hours, but his death certificate has not been signed, because we've learned his doctor refuses to do so.

Our sources say Garry's primary physician has repeatedly refused to sign because he has no idea how or why his patient died. The doctor says he hadn't seen Shandling in more than a year and as far as he knew Garry was in good heath.

The doctor's refusal to sign triggered an official death investigation by the L.A. County Coroner, who will now officially determine cause of death.

We're told the coroner's investigators have already taken blood work from the ER where Shandling was pronounced dead. The investigators have also subpoenaed records from the doctor.