The daughter of a Hollywood mob boss has shared a chilling revelation from her father that offers new insight into one of America's most notorious unsolved murders.

Luellen Smiley is the daughter of mobster Allen Smiley – a member of a loose alliance between the Italian and Jewish mafias known as National Crime Syndicate, which dominated organized crime in the US in the early to mid-20th century.

Smiley was also the associate, confidant and best friend of Benjamin 'Bugsy' Siegel, the mogul who built the Flamingo, Las Vegas's first luxury casino, and whose unsolved 1947 murder has baffled historians and law enforcement for nearly 80 years.

He was next to Bugsy when he was shot through a window of his mistress's $17 million Beverly Hills mansion – murdered in cold blood with two gunshots to the head on June 20, 1947. His killer has never been found.

Speaking exclusively to the Daily Mail, Luellen recounted the 1983 deathbed conversation she had with her father that revealed just how close he had been to Bugsy and throws open new questions about the intended target of the hit. 

'He was in the hospital. He was dying of liver failure,' Luellen explained. 

'And he said, "There's going to be a lot of talk about me after I pass. And you'll read things in the newspaper about me. Just remember that [Bugsy] was my best friend, and he would take a bullet for me."'

Luellen said she was certain that her father knew who Bugsy's killer was but he never told her, police or the FBI, despite law enforcement's persistent efforts to get him to talk.

The mobster's daughter said she believes the Italian mafia was ultimately responsible for Bugsy's assassination and that they were motivated by a combination of jealousy and a desire to control the national race wire.

The race wire was an illegal bookie network that made it possible to place off-track bets on horse races from anywhere in the US. 

In 1946, the illicit business made as much as $25,000 a month for Bugsy, equivalent to about $420,000 today. 

Luellen gave a complex description of her father as someone who was overprotective, 'secretive' and 'tyrannical' but also 'devoted,' 'charismatic' and 'very funny.' 

She claimed that other mobsters were jealous of Bugsy because he ran with celebrities. 

'He took over Hollywood,' she said. 'None of these gangsters could roll with Clark Gable and all of these movie stars, and he rolled with them.'

Her view that the Italian mob ordered the hit was also corroborated by Bugsy's daughter, Millicent Siegel. 

'I asked [Millicent] what she thought, and she said straight out: the Italian mob,' Luellen told the Daily Mail. 'She called it the spaghetti mob.'

She added that Millicent had also been 'very, very protected' while growing up and was unaware of her father's criminal activities at the time.   

Luellen spent decades researching her parents and writing a book about her family's lives titled Cradle of Crime: Confessions of a Mob Daughter, which was published in 2016.

With time and perspective earned through all those years of research and writing, she has landed at a gracious place of understanding for her father. 

She told the Daily Mail that movies and series such as The Godfather and The Sopranos did a good job of portraying the way that gangsters kept their professional and domestic lives siloed.

It was only in her 30s that she began to accept who her father was after seeing a documentary about Bugsy's murder that implicated him.

'That's when it hit me – when I actually saw it on the screen,' Luellen said. 'They showed a picture of my father. I came to terms with it then.'

All of a sudden, old memories began to click. Her father's secrecy and paranoia, the divorce of her parents, and the 'uncles' with no last names who would regularly visit her father's home in Bel Air – she saw it all in a new light.

Luellen said that growing up she would often call friends from phone booths because otherwise her father would listen in. He would also follow her in his El Dorado 'no matter where I went,' she added. 

'He was terrified – terrified that they would get to me,' Luellen said. 

During her teenage years, Luellen's father was understandably paranoid. He was being investigated by the FBI, 'who were always there' and 'wired our apartment.' 

Smiley was simultaneously worried that the mob would order a hit on him out of fear that he might speak to authorities.

Luellen's mother, Lucille Casey, was a stark contrast to her father. She was a John Robert Powers agency model and MGM musical actress who she described as 'reserved... poised, serene, gentle, very quiet [and] Catholic.' 

'Why would she marry a gangster?' she asked. 'That was the juxtaposition that I just couldn't make sense of.' 

She doesn't think that mob life suited her mother. 'She didn't want us to be around that element of society, and so I believe that's why she divorced him.'

Luellen was 13 years old when her mother died in 1966. She subsequently moved in with her father.

'Our house was sort of like the meeting place when somebody came into town, and they would congregate there,' she said.

'But none of this was obvious to me because they would go out to lunch, they'd go out to dinner, they'd invite me,' she said. 

'They never had a last name – this is Uncle Joe, Uncle Charlie, Uncle Meyer. So it was a very clandestine teenage life.'

Uncle Meyer, as it turned out, was the infamous gangster Meyer Lansky, who co-founded the Jewish-American Bugs & Meyer Mob with Bugsy and eventually became one of the leaders of the National Crime Syndicate.

Three mob historians previously interviewed by the Daily Mail all believed that Lansky would have had to approve Bugsy's assassination.

But Luellen is not so convinced. 

'Meyer, three days before the hit, went to see [Bugsy] and said, "You got to settle down. You've got to let go of the wire." Whatever advice he gave, he warned him,' she said. 

'Meyer didn't say okay - I'm sure Meyer resisted that.'

Still, she conceded that, in the end, Lansky may have had no choice but to accept the hit on Bugsy, but believes that people pushing for the assassination would have been in the Italian mob.

Luellen also dismissed another, less-accepted theory: that her father set his friend up. 

Bugsy was shot dead by a gunman standing 14ft outside the living room window of his mistress Virginia Hill's mansion in Beverly Hills. 

That night, Smiley was sitting next to Bugsy on the sofa, and he narrowly avoided also being killed by diving under the coffee table. A bullet penetrated his suit jacket as he ducked. 

'Nobody can convince me that my father would be sitting five inches away – I mean, he was so close on that little sofa. There's no way he would have known' Luellen said.

She believes that her father was actually also an intended target of the hit because he and Bugsy were best friends, and mob leadership would have been worried that Smiley would become a rat after Bugsy was killed.

Smiley would have had plenty of damaging information because, 'whatever [Bugsy] was up to, what he was doing, who he was going after or trying to swindle or whatever crime it was, he would have confided in my dad,' she said. 

Her father, she said, was born in New York and believed he did not have any choice but to join the mob. 

'Three million immigrants – Italian, Jewish – came to America, they threw them in the Lower East Side, and they said, "Fend for yourselves." This is how the gang started.

'Everyone I spoke to that grew up in that area said they were wonderful, they made [them feel] safe. They had no money, no education. My father didn't even get past eighth grade.

'They started as protection, and it went from one thing to another, from protection, from gambling into prohibition. And they got so big... they bought judges; they bought everybody. 

'They didn't come over here as criminals – they were teenagers. So that part of the story has never really been highlighted. People think that they just became gangsters because it was a great thing to do. 

'They didn't have any other choices at that time,' Luellen concluded. 'Very few did.'