CHILLING NEW CLAIMS! Did Charles Manson take even more sinister secrets to his grave? Sharon Tate’s sister drops a haunting bombshell about the cult’s true body count, warning the public that the world has only seen THE TIP OF THE ICEBERG

The Manson family killings were as gruesome as they were 𝓈𝒽𝓸𝒸𝓀𝒾𝓃𝑔. In the early hours of Aug. 9, 1969, Charles Manson’s cult followers entered the Los Angeles home of film director Roman Polanski and savagely stabbed his pregnant wife, actress Sharon Tate, and shot and stabbed four others.

The slaughter continued the following night when the bodies of grocer Leno LaBianca and his wife Rosemary were discovered inside their home. Both were stabbed multiple times.

In total, Manson and his followers were convicted of nine killings. Even today, almost 50 years later, the murders still shock — and some believe the cult was responsible for even more killings.

Sharon Tate’s sister Debra Tate is one of those people, citing several unsolved stabbing murders she believes Manson is responsible for and telling PEOPLE, “We are just scraping the surface.”

The unsolved murders are explored in the latest episode of People Magazine Investigates. Titled “Who Killed Jane Doe #59?,” the episode airs tonight at 8 p.m. ET on Investigation Discovery. (An exclusive clip is shown above.)

AP/REX/Shutterstock

Tate believes additional bodies may be buried at Barker Ranch, where the Manson family lived before their arrests in the fall of 1969.

Speaking about the unsolved stabbing murders, she tells PEOPLE, “There was nobody else at that time — other than the Zodiac Killer — who was prevalent at wielding a knife like these people.”

Bob Penn/MGM/Kobal/REX/Shutterstock

One of the stabbing victims was 19-year-old Reet Jurvetson, who was found on Nov. 16, 1969 in dense brush off L.A.’s Mulholland Drive. The knife attack and where her body was dumped — about five miles away from Polanski’s — fueled speculation that the Manson family was involved.

Tate, who has spent a lifetime attending parole hearings of Manson and his followers and fighting against their release, hoped that the release of hours of audio recordings between Manson henchman Charles “Tex” Watson and his former attorney in 1969 would shed light on the unsolved murders. But the Los Angeles District Attorney’s Office has not released the tapes.

“We might be able to rule in [additional murders] if we got our hands on those tapes,” Tate says.

Reet Jurvetson.LAPD

However, Manson Family follower Bruce Davis’ attorney Michael Beckman says the tapes are likely not incriminating.

“They haven’t used it to prosecute anybody because there’s nothing on them,” he says.

Despite this, Tate, a victims’ advocate, hopes to one day find answers.

“All of [the possible victims] have family members in various walks of life that are in a state of torture, absolute torture,” to find answers, she says.

People Magazine Investigates: Who Killed Jane Doe #59? airs tonight at 8 p.m. ET on Investigation Discovery.