Why No Charges Yet?

Alright, I'm trying not to give explicit details but I'm hoping someone can help me. This should now be simple but it's....not.

Defendant was arrested after an accident, charged with DUI and driving with suspended license among other charges. Second occurrence so automatic jail time for this place in USA. Was not in their own car, nor using their own phone. Both belonged to family member.

After being in custody, defendant confessed to a murder. Plaintiff had already called for a welfare check on the family member whose phone a car were taken.

Family member was discovered at the bottom of stairs, believed to have fallen. Did not survive, and because of confession body was sent for autopsy.

ME discovered multiple fractures in arms, legs, back. Also identified blunt force trauma to the head. Inconsistent with a simple fall but no definitive homicide stated (at least, we don't have the actual ME report yet).

Plaintiff interviewed about reasons for welfare check and informed by detectives they were actively investigating. Most info directly from them.

SO, with the defendant still in custody for DUI and other charges, why is it they have not charged with homicide? At what point would they if there is substantial enough evidence from ME to rule possible suspicious activity?