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For nearly 50 years, the family of Mary Jo Kopechne has wondered about what really happened on July 18, 1969, the night she was killed after a car driven by Sen. Ted Kennedy crashed off a bridge on the tiny Massachusetts island of Chappaquiddick.

Over the years, the information was always sparse and incomplete.

Then in recent months, Mary Jo’s cousin, Georgetta Potoski, and her son, William Nelson, received a letter. It was written by a man they did not know — a man who entitled his letter “The Untold Story of Chappaquiddick.”

Georgetta reveals the contents of the letter in Episode 7 of Cover-Up, PEOPLE’s podcast series exploring the mysterious death of Mary Jo Kopechne, a 28-year-old political aide to Bobby Kennedy.

Senator Ted Kennedy Campaigning for President
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According to Potoski, the letter writer (who did not want to participate in PEOPLE’s podcast) and his wife had a revealing conversation 30 years ago, over lunch and margaritas, with his wife’s friend, one of the women who attended the reunion party on the night of July 18, 1969 on Chappaquiddick Island.

The letter writer referred to the woman by a pseudonym, “Betty,” to protect her identity. He said Betty had had a romantic relationship with Bobby Kennedy in the months leading up to his assassination on June 5, 1968, and had worked for his presidential campaign alongside Mary Jo.

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According to the author of the letter, “Betty” said that on the night of the fatal accident, Kopechne wasn’t feeling well and fell asleep in the back of Kennedy’s car after having too much to drink at the reunion party.

“Betty got her to lie down in the back seat of the Oldsmobile and [she] went to sleep,” says Potoski. “The woman returned then to the party and subsequently said she went to bed herself.”

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According to Potoski, the letter said that “Ted got into the automobile with another woman and they drove off to do whatever and they did not know that Mary Jo was passed out in the backseat of the car.”

“After Ted and his female companion left the party, they were witnessed by Huck Look, who saw them that evening,” Potoski says in the episode. “But after they realized that the deputy sheriff saw them they sped off down toward the bridge. That was when the car went off the bridge. Ted and his female companion both escaped the car, and they went back to the party after that.”

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According to Potoski, reading from the letter, “When Betty woke up the following morning, she heard of Teddy and his tryst’s eventful escapade and asked how Mary Jo fared, to all their surprise. Until that moment, no one but Betty knew of Mary Jo’s presence in Teddy’s car.”

“When she conveyed and confirmed her story, the Kennedy damage control kicked in and informed the shocked senator.”

Spectators Look At Kennedy Car Accident
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Shortly after, the next morning, Kennedy reported the accident to the Edgartown police, saying he and his passenger Mary Jo Kopechne had gone off the bridge and that he had attempted to try and save her.

In closing, the letter writer told Potoski that he was sorry he had not had a chance to tell this to Kopechne’s parents, Gwen and Joe Kopechne, who died in 2007 and 2003, respectively, and hoped it brought some “peace and closure” to her family.

If true, the letter explains a mystery that has lasted nearly 50 years.

PEOPLE reached out to a spokesperson for Kennedy’s widow, Victoria Reggie, for comment last week, and has yet to hear a response.

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But a friend of Senator Kennedy’s who worked on Kennedy’s 1980 presidential bid says the senator once told him that he did not know Kopechne was asleep in the back seat of his car. “He said, ‘I had no idea she was asleep in the back,’ ” says the friend.

The friend says Mary Jo’s death haunted Ted throughout his life. “He felt like he let everybody down in Chappaquiddick. Teddy’s point of view was, he did something immensely stupid and he would have to live with it. In the same way that he took the deaths of his brothers on his shoulders, he took that. He is a guy who carried a lot of weight. And how well he carried the enormous burden of history.”

After Chappaquiddick,Ted Kennedy went on to have a lasting influence as a legislator in his 46-year Senate career.

After Kennedy’s death at 77 on August 25, 2009, President Obama eulogized the senator at his funeral, hailing him as “the soul of the Democratic Party and the lion of the U.S. Senate – a man whose name graces nearly one thousand laws, and who penned more than three hundred himself.”

Less than a month after his death, Kennedy’s memoir, True Compass, was released.

In it, he wrote, “That night on Chappaquiddick Island ended in a horrible tragedy that haunts me every day of my life. I had suffered sudden and violent loss far too many times, but this night was different. This night I was responsible. … Yes, it was an accident. But that doesn’t erase the fact that I had caused an innocent woman’s death.”

Now, nearly 50 years after Kopechne’s death, her cousin Georgetta Potoski still has questions about the night. She and her son don’t feel the mysterious letter explained everything.

“Until we know the truth we can never rest in peace. The truth, even if it’s not what you want to hear, has some dignity to it. It’s real, it’s what you know you have to deal with,” Potoski says.

But yet she adds, “The bottom line is I know Mary Jo is okay, I know she’s all right. The only thing I can do at this point is put them all in God’s hands. Whatever happened, whatever anybody did, he’ll have to sort it out. I can’t sort it out. We have to let it go.”

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