HOUSTON, Texas (KTRK) — Some of Houston’s grizzliest murders may be crimes you’ve never heard of. In 1979, a series of horrific killings terrified the city and left detectives baffled. Five young people were violently attacked – and in some cases, their bodies were left headless.

For more than 45 years, the families of those five victims have lived with the heartache of not knowing who killed their loved ones in such a gruesome way.

Sister Margaret Ann Calcutta says she is still searching for answers in her sister Mary’s murder. Mary Calcutta moved to Houston from Pittsburgh in the late 1970s, looking for adventure.

“Mary was always very happy, you know,” recalled Sister Margaret Ann. “We used to call her princess.”

27-year-old Mary accepted a clerk job in Houston and got her own place at the Orchard Apartments, a sprawling complex that catered to young, single working professionals. But in July of 1979, her best friend in Pittsburgh remembers getting a frantic call from her.

“It popped right out of my mouth – why are you calling me on a weeknight?” said Beth McKinstry, who was Mary’s best friend. “She explained that there was a murder in her building and she was afraid.”

A couple floors below Mary, a 33-year-old single mom, Alys Rankin, had been found stabbed to death and sexually assaulted. But perhaps most disturbing, she had been decapitated – and her head was missing.

Beth McKinstry recalls how frightened Mary sounded on the phone.

“We talked about all the precautions that she could take,” she said. “We talked about not letting anybody in that you didn’t know. We talked about walking the halls and making sure you look over your shoulder.”

But two weeks after Alys’s murder, police were called to another gruesome scene at the Orchard Apartments – this time, at Mary’s place. Mary’s body was found on her bathroom floor. She had been stabbed to death so many times that she was nearly decapitated. Police say she, too, had been sexually assaulted.

“I couldn’t process it,” said Beth. “But I remember that the detective called me and asked me some questions about Mary. Would Mary open the door for anybody that she didn’t know? I said absolutely not. I said she wouldn’t do that on a normal basis. And especially after we talked about that. I’m sure she wouldn’t do that.”

But, just seven hours before officers showed up at Mary’s apartment, there was another young woman violently attacked and killed. Doris Armstrong Threadgill was stabbed to death in her townhome, just nine miles north of the Orchard Apartments. The 26-year-old’s throat was deeply slashed, to the point that she was nearly decapitated. However, investigators noticed some differences this time. There were no signs of a struggle, like there were in Alys and Mary’s murders, and Doris wasn’t sexually assaulted.

“Either she made herself a very easy target or she knew the killer,” said HPD Detective Richard Rodriguez.

Then, two months later, another pair of violent slayings captured headlines. 18-year-old Bobby Spangenberger and 17-year-old Joann Huffman had been boyfriend and girlfriend for less than a year, when they were murdered in October 1979. Their bodies were dumped a few miles from each other in northwest Houston. Joann had been shot to death and her body was discovered in a park. Bobby’s body was found in the trunk of a vehicle at a used car lot – and his head was missing.

“Our whole lives changed,” said Brenda Spangenberger, Bobby’s sister. “I mean, completely and totally in a second.”

Today, detectives are still trying to determine if all five murders are connected.

“I think it’s highly irregular that you have multiple suspects all in the same area decapitating people at the same time,” said Detective Rodriguez. “But I’m not ready to say it’s the same individual.”

Detective Rodriguez is examining crime scene photos and witness statements. But the missing piece in this case is forensics and all the advancements in DNA over the past four decades.

“With the technology we have today, there is a whole lot there,” he said.

In our latest episode of Texas True Crime, we take you inside the evidence room in this case, for an exclusive look at what police collected and which items could possibly be tested for DNA. Plus, how Sister Margaret Ann Calcutta is working alongside a retired detective who vows to keep searching for answers in her sister Mary’s death.

“Texas True Crime: Decapitation Murders” is now streaming on the ABC13 app.

Watch Texas True Crime on your favorite streaming devices, like Roku, FireTV, AppleTV and GoogleTV. Just search “ABC13 Houston.”