Decades before a president could tweet and anyone could see minutes-old celebrity party photos on the internet, an affair between President John F. Kennedy and iconic actress Marilyn Monroe was only a rumor. Close friends of one or the other did confirm that JFK and Marilyn crossed paths publicly on at least three occasions. The first was the Waldorf-Astoria Hotel Paris Ball in April 1957, though they weren’t formally introduced. The second occasion brought them both to a party in Santa Monica hosted by actor Peter Lawford. The third was the President’s 45th birthday party in 1962. But despite the scant information about their interactions, theories about them having an affair have persisted more than half a century since the two died within months of each other.

The foundation of the JFK and Marilyn affair rumors

By the time they were thought to have connected, Marilyn Monroe was already a cinema sex symbol with movies like The Seven Year Itch to her credit. Offscreen, though, she got divorced three times and was unhappy that she never became a mom. The second and third divorces, from baseball great Joe DiMaggio in 1955 and from playwright Arthur Miller in 1961 (which made her the subject of an FBI investigation), happened during the same period she and Kennedy were occasionally moving in the same spheres.

For his part, JFK, the nation’s 45th and the first Catholic president, was known as a philanderer. While the stories about his bedding anyone from White House interns, to lusting after journalists, to actresses to adult entertainer Tempest Storm were documented and widely circulated after his death, during his presidency they were not publicized. Even in light of these other affairs, some with the full knowledge of his wife Jackie Kennedy, the possibility that JFK and Marilyn had an affair is the one that still captures the American imagination to this day.

Happy birthday, Mr. President

Those determined to see Marilyn Monroe as a presidential love interest got plenty of reinforcement one night in New York City. Monroe showed up at a Kennedy fundraiser held at Madison Square Garden on May 19, 1962—ten days before his 45th birthday. She wore a Jean Louis “barely there” dress that she’d been literally sewn into. It was breathtaking, and by the day’s standards, scandalous (even by today’s…pretty suggestive). The dress cost Monroe $12,000 and showed off 6,000 rhinestones sewn over a backless, nude-colored gown.

Monroe brought down the house with a sultry, breathy version of “Happy birthday, Mr. President” that became a sex symbol performance for the ages — often imitated, sometimes parodied and never duplicated. Speculations about the nature of her relationship with JFK got rolling that night and have never ceased. June 1 of that year Monroe turned 36. A few months later, she was dead.

The conspiracy theory that dogged the two after death

The mysterious deaths of both JFK and Marilyn fueled rumors of their affair. Monroe was the first to die, found dead by her maid lying naked and dead on August 5, 1962. The postmortem showing cause of death as barbiturate overdose. But some conspiracy theorists believe to this day that Monroe was assassinated. While it’s not as popular as the idea that Kennedy assassin Lee Harvey Oswald did not act alone, many still claim Monroe was killed by the Kennedy family to cover up her relationship with President John F. Kennedy.

JFK’s presidency was cut short just 15 months after Marilyn Monroe’s death. No one links his death to his alleged affair with her, though. Instead, once the conspiracies got going as a way to explain his assassination, many theorists started tying them to Monroe’s earlier death.

How history views a possible JFK and Marilyn affair

Fifty years after Monroe’s death, the idea that she hadn’t committed suicide and that someone else may have induced her fatal overdose persisted, though she was known for insomnia and being addicted. Some people simply couldn’t believe the beauty, who had recently been re-hired on to a film, could want to end her life. In 2012 coverage, People magazine brought fresh interviews to bear. They shed light on both the blonde bombshell’s state of mind when she died and led to new pronouncements about the possibility of her affair with JFK.

One source quoted, biographer Donald Spoto, indicated that JFK and Marilyn met four times between October 1961 and August 1962. Her masseur Ralph Roberts told the publication the two had a single “sexual encounter” that took place March 24 at crooner and actor Bing Crosby’s house.

Another revelation that helped substantiate some involvement between Kennedys and Monroe came from JFK files released in 2016. They included an FBI warning to JFK’s brother Bobby that said an upcoming book would say he (Bobby) and Marilyn had an affair.

Another biographer, James Spada, told People it was “pretty clear” that Monroe had sexual relations with both Kennedy brothers. At the same time, he undermined theories that a JFK affair led to Marilyn’s death, saying that neither affair had any impact on her overdose.

But these documents will probably do little to allay conspiracy theories about either celebrity’s untimely death or relationship with the other. Nor does the fascination with JFK and Marilyn together or separately appear to be going anywhere. In 2016, for example, that “Happy birthday, Mr. President” gown went on the auction block for a second time. While it was predicted pre-auction to sell for about $3 million, it instead brought $4.8 million, the most expensive dress ever sold at auction. As for the previous record holder, that was a Monroe gown, too: The puffy skirt she wore in the iconic steam vent still from The Seven Year Itch, which brought $4.6 million in 2011.