Los Angeles (E! Online) What do Michael Jackson and Albert Einstein have in common...besides pure genius? Both their brains are in a glass jarat least for the time being.

The Los Angeles County Coroner's Office says it's holding onto a section of the late King of Pop's brain as part of its investigation into how he died.

Because toxicology tests take several weeks and are not yet complete, a coroner spokesman says pathologists need to wait for the organ to harden before they can conduct neuropathology tests to help determine what caused Jackson's apparent cardiac arrest and subsequent death.

Investigators have focused on the possibility that Jackson accidentally overdosed on a potent anesthetic, and a thorough examination of brain tissue might shed some light on what drugs were in his system.

The coroner's office has told Jackson relatives they have the option of burying Michael without the missing portion of his brain, or they can wait until tests are done and the tissue returned to its original place before finally laying him to rest.

That could explain why relatives have yet to hold a burial service for the late pop star.

The whereabouts of his remains are not known, having disappeared from public view following yesterday's poignant public memorial; however, a family friend told E! News Michael would be back at Forest Lawn Memorial Park, where a private funeral service was held earlier Tuesday.

Jackson's death certificate, signed by his sister La Toya, was also issued Tuesday, listing the cause of death as "deferred." A burial permit filed with the Los Angeles County's Office of Vital Records has not been made public