Image copyright PA Image caption Police say Robert Wagner was the last person to see his wife, actress Natalie Wood, alive

US police investigating the mysterious death of film star Natalie Wood say her husband Robert Wagner is now a person of interest.

Investigators in Los Angeles say Wagner has "constantly changed his story", adding that his version of events does not "add up".

Wood was found drowned in 1981 during a California boat trip with Wagner and the actor Christopher Walken.

Her death was ruled to have been an accident but questions have lingered.

"As we've investigated the case over the last six years, I think [Wagner] is more of a person of interest now," Lt John Corina of the Los Angeles County Sheriff's office told CBS News.

"We know now that he was the last person to be with Natalie before she disappeared," he said. "I haven't seen him tell the details that match all the other witnesses in this case".

"I think he's constantly changed his story a little bit. And his version of events just don't add up," Lt Corina added.

Conflicting versions of what happened on the yacht have contributed to the mystery of how the actress died in November 1981.

Family members have previously asked for authorities to re-examine the findings of the original investigation.

Image copyright Getty Images Image caption Wood, who was 43 when she died, was nominated for an Oscar three times but never won

Wood, who starred in West Side Story and Rebel Without a Cause, had been partying the night before her death. The coroner's investigation ruled she had been drinking and may have slipped trying to board a dinghy.

But police say two new witnesses have corroborated accounts of a fight between Wagner and Wood on the night she disappeared.

They say it appears the actress was the victim of an assault. The post-mortem report said Wood had bruises on her body and arms as well as a facial abrasion on her left cheek.

The captain of the boat, Dennis Davern, told NBC News in 2011 that he lied to police during the initial investigation and that a fight between Wood and Wagner had led to her death.

Image caption Wagner has had numerous TV and film roles

TV star Wagner, 87, has not commented on the latest developments but acknowledged in his memoir that he had argued with his wife before she disappeared.

Investigators say he has refused to speak to them since the case was reopened in 2011.

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In 2012, Wood's death certificate was amended to reflect the uncertainty surrounding her death. It now says she died as a result of "drowning and other undetermined factors" and the circumstances of how she ended up in the water are "not clearly established".

As a child, Wood featured in films like Miracle on 34th Street and The Ghost and Mrs Muir.

She was nominated for a best supporting actress Oscar for her role in Rebel Without a Cause, and for best actress for Splendor in the Grass and Love with the Proper Stranger.