The sister of Hollywood actress Natalie Wood claims police investigating her mysterious death have unearthed two new witnesses and are close to solving the case, Daily Mail Online can reveal.

Lana Wood says cold case homicide detectives have made 'substantial' inroads in the unexplained death of her Hollywood star sibling.

This comes as Lana revealed she has received telephone death threats to 'lay off' Natalie's then-husband Robert Wagner - who she believes bears responsibility for her sister's death and knows more than he's told.

Lana has slammed film star Wagner for not cooperating with the police, claiming he holds the key to Natalie's 1981 death.

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Lana Wood (right) says cold case homicide detectives in Los Angeles have made progress in the unexplained 1981 death of her sister, Natalie Wood (left)

Natalie Wood drowned aged 43 after going missing during a boat trip to Santa Catalina Island off the coast of California. Pictured above, she and husband Robert Wagner (left) are seen on their boat Splendor, just weeks before she died in 1981

Wagner allegedly said he thought Natalie was 'screwing around' on the boat before she died. Pictured above the couple enjoys a day out on the Splendor just weeks before she died

Lana, 70, claims that Los Angeles detectives also confided in her that they are 'very close' to making an arrest in the case

Speaking to Daily Mail Online, Lana said: 'The police told me that they have found new witnesses who were not willing to speak back then.

'These witnesses have been helping police but they won't go on record or testify, they don't want to be heavily involved in the investigation which is a shame.'

Lana, 70, claims detectives also confided in her that they are 'very close' to making an arrest in the case - but the Los Angeles District Attorney's office has insisted more evidence is needed to press charges.

Lana added: 'It's very frustrating, they have moved the case forward and made substantial inroads, but still can't press charges.

'After 35 years I feel we're so close to finding out what happened to my sister but the LA Attorney's Office is refusing to press charges against Robert Wagner - the man I believe is responsible. It's a cover-up and I won't rest until Natalie gets justice.'

Three-time Oscar nominee Natalie, who starred in Miracle on 34th Street as a child star, Rebel Without a Cause as a teen and then with Warren Beatty in Splendor in the Grass, drowned at age 43 after going missing during a boat trip to Santa Catalina Island off the coast of California.

She was with Wagner, actor Christopher Walken and the boat's captain, Dennis Davern on the evening of November 28, 1981.

Natalie and Wagner married twice - the first time in 1957 and stayed married until 1962 and then in 1972 until her untimely death. They were considered the most romantic of Hollywood couples.

Natalie's death was originally ruled 'an accidental drowning' by a coroner at the time.

But in 2012, the cause-of-death ruling was changed to 'drowning and other undetermined factors', and cited strange bruises found on the actress' body.

For Lana, who lives in Fillmore, California, her former brother-in-law still has lots of questions to answer.

Natalie was with her husband Robert Wagner (pictured above with Natalie), actor Christopher Walken and the boat's captain, Dennis Davern on the evening of November 28, 1981

Natalie's (pictured above with Wagner) death was originally ruled 'an accidental drowning' by a coroner at the time

But in 2012, the cause-of-death ruling was changed to 'drowning and other undetermined factors', and cited strange bruises found on the actress' body. Natalie is pictured above with Wagner and their two children, Natasha (center) now 45, and Courtney (right) in 1976. She is 42

This is the yacht Splendor on which Natalie and Wagner came to Catalina for the holiday weekend. Natalie's body was found floating 200 yards off Blue Cavern Point (to the rear of the yacht) after she was reported missing

Wagner bends over to kiss flower covered casket of his wife, Natalie, during graveside ceremonies for her at Westwood Memorial Park Cemetery in 1981

Christopher Walken, pictured above with Natalie while filming for Brainstorm, was on the boat when Natalie went overboard

The yacht's former captain Dennis Davern has blamed Robert Wagner for Natalie Woods' death. He said Wagner did not let them do anything to find her

She says: 'I've always believed Natalie was killed, nothing has come up to change my mind on that.

'It's easier for me to believe RJ (Wagner) has nothing to do with it, because the other possibility is heart-breaking, heart-wrenching, horrendous, horrific, that this man that she had known and was married to twice, might be responsible, it's horrendous for me to even imagine that.

'I would prefer to believe the other, but unfortunately it doesn't point that way.'

Earlier this year Lana confronted Wagner at a charity luncheon his wife actress Jill St. John [she starred with Wagner in the long-running 80's TV series Hart to Hart and they married in 1990] was holding in Beverly Hills.

She was caught on video quizzing the actor over her sister's death and his apparent lack of interest in helping the police since the case was reopened two years ago.

'RJ has never spoken to them and I don't know why he continues to insist on not doing so, unless he's going by the old thing of, 'I spoke to them 35 years ago,' she says. 'It's true he spoke to the coastguard and the sheriff when they arrived on the boat but he has refused to speak to them since then.

'If he has nothing to hide why wouldn't he speak to them now? I put myself in the same situation, why I wanted to give every bit of information I could if that's what they require.

'So that makes it unpleasant and suspicious.'

Lana says she wasn't invited to the event but knew Wagner was going to be there so waited outside.

She says: 'All these years I have spent so much time saying I don't have any ill will towards RJ, I don't believe in vengeance, I believe things happen for a reason, when people are naughty they will get their comeuppance, but it's not for me, I do not want to inflict pain or discomfort on anyone, particularly on Natalie's children.

'But after speaking to RJ for that brief time he is not the person I knew, he wouldn't look me in the eye. I always found him to be shallow, but he's changed from the warm, loving man he was when he was married to Natalie, he was very dear and a very good father

Natalie, pictured on her wedding day second from left, with her mother, Maria Gurdin (left), Wagner's mother, Chat Wagner (second right) and sister Lana, died in November 1982

Wagner and Natalie are pictured (center) together at their first wedding, with, from left to right, Wagner's parents, Lana and Natalie's parents. They were married in 1957 and then again in 1972 until her death

'But I just found him incredibly hateful and he just wasn't listening to me and I was so upset I just couldn't bring around the conversation.'

The awkward confrontation was caught on camera, which Lana says 'spoiled everything'.

Lana says: 'There were things I wanted to ask him, there were things I wanted to say and that spoiled everything.

'What is up in the air is exactly how Natalie died which has never been told by anyone at any time.'

Lana claims detectives failed to take finger nail clippings from Natalie following her death - something, she says, is standard law enforcement procedure.

A rape kit exam was also not done.

'According to the detectives on the case they say a lot of things were missing, things that were not done,' she said.

While Lana is convinced Wagner is in someway responsible for her sister's death she believes his actions weren't premeditated.

She explains: 'People ask me if I think it was an accident. Well I think it was an accident in that it was not pre-meditated so the difference is, was he responsible, absolutely without question was he responsible, was it premeditated, I don't believe that for an instant.

'There was too much going on all at once and it led to some pretty unfortunate things to put it mildly.'

Lana recalls how she tried to get answers from Wagner shortly after her sister's funeral.

The family held a wake at the home Wagner shared with Natalie, and Lana went to console her brother-in-law and ask him what happened.

She recalls: 'RJ went up the stairs and I said to someone I'm going to talk to RJ. I went upstairs and he was sitting on the edge of the bed and I said, "RJ, what happened, you've simply got to tell me what happened?" and he said, "I'm sorry".

Natalie, pictured left in a childhood photo and right as a baby with her father, drowned mysteriously when she was just 43 years old

For Lana (left), who lives in Fillmore, California, her former brother-in-law still has lots of questions to answer in connection to Natalie's (right) death

Lana said she has received telephone death threats to 'lay off' Natalie's then-husband Robert Wagner

'Then someone grabbed my arm and said leave him alone don't talk to him now, and I said "fine". And that was it, so "I'm sorry" doesn't quite tell me what happened.

'I get what happened from Dennis Davern, who was the captain on the boat, I get what happened from the detectives who were on the case, I get what happened from everybody but him (Wagner).

'And he's the only one who really knows, he was there when Natalie was overboard.'

According to the boat's captain Dennis Davern, that night Natalie, husband Wagner and Christopher Walken had dinner and drinks at Doug's Harbor Reef and then continued drinking aboard the boat, where Wagner and Walken began arguing.

During the argument, Walken went to bed as did Natalie and when Wagner came down to say goodnight a short while later, she was gone.

Speculation has it that she accidentally slipped and fell overboard. However, others have speculated that a lover's quarrel between Natalie and Wagner took place in their room before the accident - a fact which Davern has since confirmed.

And in 2011, the case was reopened after Davern admitted that he had lied to investigators about the mysterious death and alleged that Woods' husband Wagner was responsible.

'I made some terrible decisions and mistakes,' Davern told NBC's Today show. 'I did lie on a report several years ago.'

'I made mistakes by not telling the honest truth in a police report.'

When asked on the Today Show whether Wagner was responsible for her Natalie's death, he responded: 'Yes, I would say so. Yes.'

It was also alleged at the time that Natalie, upset after a row with her husband, may have taken a dingy herself to return to shore and somehow fell overboard en route.

But Lana disputes this. Pointing to the fact that Natalie was found dead still wearing her nightgown, she says: 'Do you know that Natalie would not even pick up the mail without being completely dressed and made up, she went nowhere like that.

Natalie (left) was eight years older than her younger sister Lana, who is still searching for answers in her sibling's mysterious death

Lana (pcictured with Natalie in the 1960s) recalls how she tried to get answers from Wagner shortly after her sister's funeral, but he would only say, 'I'm sorry'

Lana (pictured with Natalie) says her sister's death affected the whole family and that she was blacklisted from Hollywood shortly after

'She was also terrified of water, she had a swimming pool but never got in it, she did not get in her own pool and swim.

'So the idea that she took out a dingy by herself, dressed in only her nightgown is absolutely nonsense.'

Lana says Natalie's death affected the whole family.

'It not only destroyed and ended Natalie's life, it hurt her children's lives, my life,' she said.

The former actress and film producer also accuses Wagner of black-listing her from Hollywood after Natalie drowned.

She says: 'I couldn't get work, I was in production, I was producing films, I was over at Warner Brothers.

'I couldn't get work immediately after Natalie's death. I received a call from my agent at the time and he said RJ had had me black-listed.

'Not only did he not want to bump in to me, he didn't want to answer to me and he knows I have a tendency to speak my mind. He's hurt a lot of people.'

The last time Lana says she spoke to Wagner - before the recent confrontation - was when her mother passed away 17 years ago.

'I was taking care of her in my home and she passed and I called RJ, I called him first, I don't know why but I did,' she recalls. 'He got on the phone, he was very brief and unsure of himself, I was a complete mess, he hung up the phone.

'Then the phone rang again about ten minutes later and it was RJ and he just said, "I just wanted to tell you that I'm really sorry". And that was the last time I spoke to him and that was 17 years ago.

'My mom was never the same after Natalie died.'

Wagner has vehemently denied any involvement in Natalie's death.

The star's lawyer, Blair Berk, has claimed the actor has 'fully cooperated' with police.

Wagner did not respond to Daily Mail Online's request for comment.

Natalie starred in several films, including West Side Story, in which she played the role of Maria (pictured above in a scene from the film with Richard Beymer)