The death certificate was amended on August 7 to list her death as caused by "drowning and other undetermined factors" rather than "accidental drowning," Los Angeles County Sheriff's Detective Kevin Lowe said Wednesday.
Last November, homicide investigators decided to take a new look at one of Hollywood's most enduring mysteries after they were contacted by people who said they had additional information about the actress's drowning, the sheriff's department said.
"This new information is substantial enough to make us want to take a new look at the case," Lt. John Corina said at the time.
The announcement persuaded tipsters to come forward with additional "intriguing" information, sheriff's spokesman Steve Whitmore told CNN last January.
Investigators did not comment directly on statements made in news reports in November by Dennis Davern, the captain of a yacht owned by Wood and her husband, actor Robert Wagner.
Davern offered a new account of how Wood's death was reported, saying that Wagner waited hours to call the Coast Guard after Wood went missing off Catalina Island, near the California coast, following an argument the couple had.
Authorities haven't gone into specifics about who has been interviewed, but they did say when they reopened the case that Wagner wasn't a suspect.
Wood drowned in the Pacific Ocean on November 29, 1981, off the isthmus of Catalina Island. She once said in a televised interview that her greatest fear was of dark seawater.
Her body was found floating in the water about a mile away from the yacht, in a long nightgown, socks and a down jacket, according to police reports.
The autopsy report showed the actress had two dozen bruises on her body, including a facial abrasion on her left cheek and bruises on her arms.
"My sister was not a swimmer and did not know how to swim, and she would never go to another boat or to shore dressed in a nightgown and socks," said Lana Wood, referring to theories that the actress voluntarily jumped from the boat.
Although the county coroner's office ruled at the time that Wood's death was an accident, others say the case hasn't made sense.
In 2010, Lana Wood told CNN she believes a highly charged argument between her sister and Wagner on the yacht's back deck preceded Wood's drowning. She told CNN last year she does not suspect foul play.
"I just want the truth to come out, the real story," she said.
Davern, the former captain of the yacht Splendour, broke his long silence with a detailed account of that day in "Goodbye Natalie, Goodbye Splendour," a book he wrote with his friend Marti Rulli. It was published in September 2009.
Davern has said he believes Wood's death was a direct result of a fight with Wagner.
In a lengthy interview with KTLA, Davern, who had worked for the couple for many years, admitted lying to investigators 30 years ago about some details of the case.