EXCLUSIVE

Natalie Wood's death certificate has been changed from "Accident" to "Undetermined" ... TMZ has learned.

Law enforcement sources tell TMZ ... the L.A. County Coroner filed documents making the change late last month.

Members of Wood's family tell TMZ ... detectives from the L.A. County Sheriff's Department -- which has reopened the death investigation -- paid them a visit earlier today informing them of the change. They tell us ... detectives told them the change was made in part because some of the bruises on Wood's body were inconsistent with death by drowning.

Family members add ... they were told at this point authorities can't prove definitively Wood's death was accidental or the result of foul play. Detectives did tell family members Wood's death was definitely from drowning, but the way she ended up in the water is unclear.

Wood died November 29, 1981 after an alcohol-fueled party on a boat off Catalina with Robert Wagner and Christopher Walken on board.