Petaluma Police are investigating the death of a 30-year-old woman who was brought to Petaluma Valley Hospital just after midnight Tuesday morning and later pronounced dead.

The woman, who has not been identified pending notification of her family, was unresponsive when she was brought to the hospital in a private vehicle driven by a woman who knew her, according to Lt. Tim Lyons. The driver was interviewed by police Tuesday and is being cooperative, but it is remains unclear what led up to the woman's death.

Investigators were trying to determine Tuesday where the woman was when she showed signs of distress and why emergency crews were not called before she was driven to the hospital.

"If you are in the car already, it makes sense" to drive to the hospital, Lyons said. "We are not sure where this all started."

There were no visible signs to point to a cause of death and investigators said the coroner will have to determine whether drugs or alcohol were involved, Lyons said.

It was unclear where the woman lived at the time of her death but investigators determined she did reside in Sonoma County, according to Lyons.