Actor Robert Wagner, 87, has been named a "person of interest" in the 1981 drowning death of his late wife, the actress Natalie Wood.

Though her death was ruled an accident at the time, in 2011, after the case was reopened by the Los Angeles County Sheriff's Department, the cause of death was changed from accidental drowning to "drowning and other undetermined factors."

"As we've investigated the case over the last six years, I think he's more of a person of interest now," Lieutenant John Corina told the US current affairs program 48 Hours.

"We know now that he was the last person to be with Natalie before she disappeared."

The death of Natalie Wood has become one of Hollywood's enduring narratives, in part because its lack of clear resolution has cast a long shadow over the events and people connected to it.

Wood drowned in 1981 while on a boat trip to Catalina, a small island off the California coast which is a popular destination for tourists and day trippers.

On November 28, 1981, she was aboard a boat named Splendour, with Wagner, actor Christopher Walken and the captain of the Splendour, Dennis Davern.

At 8am the following morning her body was recovered some distance from the boat, and the autopsy report revealed she had bruises on her body and arms and an abrasion on her left cheek.

In his memoir Pieces of My Heart, Wagner said he and Wood had argued before she disappeared; at the time he told investigators he went to bed alone and when he fell asleep Wood was not present.

The case was reopened in 2011 after Davern said he had lied to police during the initial investigation, alleging that Wagner was responsible for Wood's death.

"I made some terrible decisions and mistakes," Davern said in 2011. "I did lie on a report several years ago. He added, "I made mistakes by not telling the honest truth in a police report."

Davern said in 2011 that on the night of Woods' death "we didn't take any steps to see if we could locate her.

"I think it was a matter of, we're not going to look too hard, we're not going to turn on the searchlight, we're not going to notify anybody right now," Davern said.

After the investigation triggered by Davern's testimony, the Los Angeles County chief medical examiner amended Woods' death certificate, noting the circumstances of how Wood ended up in the water are "not clearly established".

Corina said in a new interview with 48 Hours that Wagner's testimony was pivotal in the case.

"I haven't seen him tell the details that match all the other witnesses in this case," Corina said. "I think he's constantly changed his story a little bit. And his version of events just don't add up."

It is understood that both Walken and Davern, who were aboard the Splendour the night of Wood's death, have spoken with police as part of the re-opened investigation but Wagner has refused.

Another investigator, Detective Ralph Hernandez, told 48 Hours that while it was unclear whether Wood's drowning was not accidental, "it's suspicious enough to make us think that something happened.

"We have not been able to prove this was a homicide and we haven't been able to prove that this was an accident either," Hernandez said. "The ultimate problem is we don't know how she ended up in the water."

The 48 Hours report, Natalie Wood: Death in Dark Water, will air in the US on Saturday.