Late on the night of July 18, 1969, a black Oldsmobile driven by U.S. Senator Edward Kennedy plunged off the Dike Bridge on the tiny island of Chappaquiddick, off Martha’s Vineyard, landing upside down in the tidal Poucha Pond. The 37-year-old Kennedy survived the crash, but the young woman riding with him in the car didn’t. Though newspaper headlines at the time identified her simply as a “blonde,” she was 28-year-old Mary Jo Kopechne, a respected political operative who had worked on the presidential campaign of Senator Kennedy’s brother, Robert Kennedy.

Kennedy later claimed he dove repeatedly “into the strong and murky current” to try and find Kopechne before making his way back to the cottage. He then drove back to the scene with his cousin, Joseph Gargan, and aide Paul Markham, who both tried in vain to reach Kopechne. But rather than report the accident to the police at that time, Kennedy returned to his hotel in Edgartown. As a result, Mary Jo Kopechne remained underwater for some nine hours until her body was recovered the next morning.

The incident at Chappaquiddick ended Kopechne’s young life and derailed Ted Kennedy’s presidential ambitions for good, but nearly half a century later, the details of what happened that fateful night remain unclear. Conspiracy theories and questions endure. How did Kennedy end up driving off the bridge? Was he drunk? What were he and Kopechne doing together that night? Was there a third person in the car? Why did he wait so long to report the accident?

In a speech the following week, Kennedy maintained he had not driven drunk, and that there was “no truth, no truth whatever to the widely circulated suspicions of immoral conduct that have been leveled at my behavior and [Kopechne’s] regarding that evening.” Kennedy attributed his actions after the accident to injury (he suffered a concussion), shock and confusion.

How long was Mary Jo Kopechne alive after the car flipped?

Kopechne likely did not die instantaneously, but her final moments remain a mystery. When John Farrar, a diver for the local fire department, found Kopechne’s body the morning after the crash, its positioning suggested she had remained alive for an unknown period of time after the car went underwater. Her face was pressed into the footwell, and her hands gripped the back of the front seat, as if she had been trying to push her head into a pocket of air.

While some observers of the case suspected she could have been saved if Kennedy had gone for help earlier, others—including James E.T. Lange and Katherine DeWitt Jr., authors of Chappaquiddick: The Real Story—maintain that the cold temperature of the water and the condition of the car made it unlikely she survived for any extended period of time.

The report from the inquest into the accident released by the Massachusetts Supreme Judicial Court in 1970 concluded that as there was no evidence any air remained in the submerged car, it wouldn’t seek or allow any testimony about how long she may have lived, as “this could only be conjecture and purely speculative.”

Who was Mary Jo Kopechne?

Kopechne, who grew up in New Jersey, had volunteered for John F. Kennedy’s presidential campaign while in college. After obtaining a business degree, she taught at a Catholic mission school in Alabama and worked for a Florida senator before getting a job in Robert Kennedy’s Senate office. During his 1968 presidential campaign, Kopechne helped write the candidate’s speeches.

The weekend of the accident, Kennedy operatives had invited Kopechne— who continued working in politics after the assassination—and five other women who had worked on the campaign to reunite on the Vineyard in recognition of their work. On the night of July 18, Kopechne and the other “boiler room girls” (as they were known) attended a cookout at a cottage on Chappaquiddick, along with Kennedy and five other men. Late in the evening, Kennedy and Kopechne left the party together.

Kennedy later stated that Kopechne felt ill and that they were headed for the ferry to Edgartown, where both were staying in different hotels. Though the fact that Kopechne left her purse and hotel room key behind at the cottage cast doubt on this claim, no evidence ever surfaced that the two had a romantic or sexual relationship.

Why didn’t Ted Kennedy go to prison?

To charge Kennedy with involuntary manslaughter, the police would have had to establish that he did something illegal, like speeding or driving under the influence. But Kennedy didn’t contact local police until 10 a.m. on the morning of July 19, after Kopechne’s body had been found in his submerged car in Poucha Pond.

Therefore, police were unable to test Kennedy’s blood alcohol level at the time of the accident, and they had no other evidence of illegal activity. In the end, Kennedy pled guilty to the charge of leaving the scene of the accident, and received a two-month jail sentence (which was suspended) and a temporary driving ban.

In addition, Kennedy’s public perception was bolstered by the immediate damage control and legal efforts undertaken by a group of Kennedy confidantes and advisers, including ex-defense secretary Robert McNamara and JFK speechwriter Ted Sorensen. These behind-the-scenes maneuverings are explored in the 2018 film Chappaquiddick, whose script is based on the historical record, including the inquest into the accident released by the Massachusetts Supreme Judicial Court in 1970. The film portrays the inner workings of the Kennedy political machine as it worked to obscure the facts of the tragic incident from the public and save Ted Kennedy’s political career from imploding.

Timing also helped: During that fateful weekend at Chappaquiddick, most Americans had their eyes glued to their TVs, watching a different drama unfold: the Moon Landing. Apollo 11 had lifted off from Cape Canaveral on July 16, 1969 and on the evening of July 20, Neil Armstrong and Buzz Aldrin took their first steps onto the moon’s surface.

How did Chappaquiddick affect Ted Kennedy’s career?

In the end, Kennedy didn’t resign from the Senate, but he did announce he wouldn’t run for president in 1972, as many supporters had been hoping he would before Chappaquiddick. The fallout from the accident (and lingering rumors of a cover-up) permanently doomed Kennedy’s presidential ambitions: In 1980, when he finally did seek the Democratic presidential nomination, he suffered an embarrassing defeat to the unpopular incumbent Jimmy Carter.

Kennedy would serve Massachusetts in the Senate for three more decades, and would become one of the nation’s most respected elder statesmen by the time of his death, of brain cancer, in 2009. In True Compass, a memoir published after his death, Kennedy wrote that his actions the night of the accident were “inexcusable,” and that he “made terrible decisions.”

He knew at the time the accident would be devastating to his family, Kennedy admitted, as well as damaging to his political career. Finally, Kennedy wrote that “my burden is nothing compared to [Kopechne’s] loss and the suffering her family had to endure. She also didn’t deserve to be linked to me in a romantic way. She deserved better than that.”