SOUTH LOS ANGELES (CBSLA) — Every year, Debra Thomas sends Christmas cards to detectives at the LAPD asking them not to forget her sister.

They haven’t, and now there’s a renewed effort to solve a murder mystery in South L.A. that is  more than 40 years old. It involves the killing of an elementary school teacher in her own home, and for the first time, her family is speaking out.

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“Who did it? Where? Why?” the victim’s sister Thomas asked.

Those are the questions Thomas has been asking ever since she found out her older sister, Francine, was murdered.

“It makes you angry; it hurts you to the core,” she said.

In 1974, Francine Moran, 26, was a LAUSD teacher at 32nd Street Elementary School in South L.A. She was also working on her master’s degree in education at USC.

“Francine was quiet, she was very smart academically, very independent.” Thomas said.

On the evening of March 6, the young teacher was preparing her lessons for the next day. Police say she had school material laid out on her bed. With the radio playing in the background, she got a knock at the door. And whoever it was, she let them in.

“No sign of forced entry, and Ms. Moran was sitting in a chair in her living room when she was shot multiple times,” LAPD detective Ricardo Feria said.

Feria took over this cold case a few years ago. Since then, he’s been trying to identify a suspect, a motive. He says there was little evidence left behind. But based on the fact Moran let her killer in and the fact she was shot several times, it must have been someone she knew.

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Neighbors hadn’t seen her in a few days, so they decided to call police. And when police got here, they found her shot inside.

“This is a very personal situation. Obviously, somebody who had some very personal feelings towards her that caused this,” Feria said. “The living room area where she was sitting was in order. Everything appeared to be, again no sign of forced entry, no sign of robbery.”

After Thomas got the news, and as police tried to find a suspect, she tried to figure out who could have done this.

“I was just shocked. It was surreal. Not Francine,” Thomas said. “I did ask her one time is there somebody in your life, and she said ‘Oh, there’s somebody, I’ll let you know if things get serious.’ So that’s all I knew.”

During the course of the investigation, no suspect or person of interest was ever named and no weapon was recovered. Feria believes there were people who had information but they didn’t talk because there was mistrust and tension between the community and police at the time.

“I believe somebody during that time would’ve heard or seen something and maybe felt nervous or fearful. But it’s been 43 years, and we’re hoping somebody can reach deep down in their heart and come forward.”

Thomas keeps her sister’s memory alive and continues to hope her sister’s case will one day no longer be cold.

“Francine, I never stopped fighting. Mom and dad may be gone, but sometimes all it takes is one person. I am fighting for her and hopefully if it is solved, she’ll tap me on the shoulder and say ‘Debbie, you did it.’ ”

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If you have any inform about this case, you are asked to call LAPD South Bureau Homicide Unsolved Section. You can remain anonymous.