On 7 November 1969, Sister Cathy Cesnik, a Catholic nun and popular schoolteacher at a Baltimore high school, went missing. Two months later, her body was found in a dump five miles from her home, with a blow to the back of her skull.

The murderer was never found.

Now new Netflix documentary series The Keepers picks up the case, speaking to dozens of Sister Cathy’s friends, former students and people who for nearly 50 years have tried to uncover what happened, who is responsible, and why the truth has never been revealed.

Find out more about the series and the story behind it below.

What happened to Sister Cathy?

Sister Cathy taught English and drama at Archbishop Keough High School, a traditional high school in Baltimore.

On the night of her disappearance, Cathy left her apartment telling her flatmate she was going out to buy a present for her cousin, who had just got engaged.

At 11pm, when Cathy had still not returned, her flatmate called two of their priest friends who came over and notified the police. Later that night, Cathy’s car was found unlocked and parked illegally a short distance from her flat. However, Cathy herself was nowhere to be found.

No, The Keepers is not "the new Making a Murderer". Here's why

Four days later, another woman, 20-year-old Joyce Malecki, was also reported missing, her car abandoned. Her body was found two days later, but Cathy’s was not discovered until nearly two months after her disappearance, on 3 January 1970.

To this day, both Cathy and Joyce’s murders remain unsolved.

Why is the case still being talked about?

In 1994, two women at the school where Cathy worked came forward with accusations of horrific child sex abuse that they suffered while they were students. The women filed a lawsuit against Father Joseph Maskell, the chaplain of Keough High School, claiming he subjected them to “repeated sexual, physical, and psychological abuse”.

In the court documents the women are identified only as ‘Jane Doe’ and ‘Jane Roe’. Other women came forward to testify to the alleged abuse at Keough High School, however, after a high profile trial the case was thrown out. A report in the Huffington Post explains that the law in Baltimore states that victims of sexual abuse have three years to file a civil lawsuit after the abuse has taken place or when they discover it. It was ruled that despite evidence the women had repressed or buried memories of the abuse, their case fell outside of the three year window.

What does this all have to do with the murder of Sister Cathy?

'Jane Doe', 16 years old in 1969, claims that Father Maskell one day took her in his car, and showed her the body of Sister Cathy in the days after her disappearance.

In the documentary, she recalls how the priest leaned over her and said, "You see what happens when you say bad things about people?"

The series features interviews with many former students who claim that Sister Cathy had become aware of the abuse going on at the school. Their theory is that she was murdered in order to prevent her from revealing the truth – and to terrify others into silence.

Maskell died in 2001, and since then 'Jane Doe' and 'Jane Roe' have revealed their true identities. They are Jean Wehner and Teresa Lancaster, and both feature prominently in The Keepers.

A small group of former students have gathered together both to offer support to the victims, and to try and find out what really happened to Sister Cathy. Two women in particular, Gemma Hoskins and Abbie Schaub, have led the investigation, setting up a Facebook page called 'Justice for Catherine Sesnik and Joyce Malecki'.

The Keepers follows their hunt for the truth, and makes its own investigations into the stories of abuse and cover-ups that surround the case of Sister Cathy.

“The story is not the nun’s killing," Abbie Schaub tells the filmmakers. "The story is the cover-up of the nun’s story.”

Since the series was announced, the story has indeed been making headlines again. As early as 4 May, the Baltimore Sun reported that police had exhumed the body of Maskell to compare his DNA with crime scene evidence.

Police spokesperson Elise Armacost said that detectives "felt very strongly that in the interest of leaving no stone unturned, it was necessary to exhume Maskell's body and compare his DNA to the evidence that is remaining." 

However, two days before the documentary was due to be released, Baltimore police announced that DNA taken from Maskell's body does not match DNA from the murder scene of Sister Cathy.

The series' release has been challenged by some in the Catholic Church in the United States: Bill Donohue, president of the Catholic League, said the documentary "will surely feed the appetite of those ready to believe the worst about the Church", called it "scurrilous" and accusing it of 'indicting without evidence'.

How did the filmmakers find out about the story of Sister Cathy?

The Keepers director Ryan White tells RadioTimes.com that he first heard about the case thanks to a family connection.

"My mum is from Baltimore, and my aunt actually went to high school where this all happened," he explains. "She went to Keough, and Sister Cathy was her English teacher in 10th grade. She was actually in the same class as ‘Jane Doe’, they were friends.

"My aunt found out who 'Jane Doe' was a few years ago, because it had always been a mystery. So she connected me with Jean, who is Jane Doe, and that’s how it all began."

However, this was just the start of a three-year-long process to bring the story to screen.

"She’s an abuse victim, a trauma victim, and so she was very wary of outsiders," he says. "It wasn’t like I sat down with her because she knew my aunt and my Mum. We spent three or four different trips to Baltimore before we ever brought a camera, having conversations about what my vision was, and what it would take for her to participate.

"She had been silent for so long, and I think she wasn’t going to commit to a project that didn’t have integrity or didn’t do her story justice," he adds.

It is this 'trust building', he suggests, that has allowed The Keepers to dig deeper than any previous investigation into who killed Sister Cathy. The series features dozens of friends, relatives, local journalists, retired and current police officers and others close to Sister Cathy.

"So much of this documentary was relationship building and trust building," he says. "It involves dozens of people; the series continues to grow and grow as you watch it and it moves into the modern day."

When is The Keepers released on Netflix?

All seven hour-long episodes will be released on the streaming service on Friday 19 May 2017, at 8am UK time.

The series is directed by Ryan White, executive produced by Jessica Hargrave, and made by Film 45 and Tripod Media for Netflix.