Just 21 years old, Diane Schofield was the life of the party, younger sister Twyla Johnson said.

“She was just so beautiful with the longest legs you ever saw. You could have 15 girls in the same outfit in the same room and everybody would look at Diane,” Twyla told Dateline.

Twyla and Diane grew up in Iowa, near Des Moines, and on July 4, 1975, they got together and spent the day with a few friends at a nearby lake.

Diane left the lake a little early to go to her job as a waitress, Twyla told Dateline. She didn’t hear from her sister for the next few days, but told Dateline, “Back then, you kind of just did your own thing,” so she didn’t find it strange to not hear from Diane.

Diane shared an apartment with friends, who grew concerned, according to Twyla, as they also had not heard from Diane. So on July 10, one of Diane’s roommates, Mike Killion, reported her missing to police.

According to Twyla, just hours later, police found Diane’s car in a steakhouse parking lot across the street from the Des Moines airport. According to local media, a parking lot sweeper had noticed an odor coming from the car and alerted authorities.

Police found Diane’s body in the trunk of the car. She had been strangled with a strip of cloth, according to a 2016 Des Moines Register article. Her ankles and hands were bound. The article states the police investigation discovered that Diane had last been seen alive at a gas station around 9:00 p.m. on July 7. Based on evidence and eyewitnesses, investigators placed Diane’s death sometime shortly after that sighting.

“I was watching TV and the news said, ‘Des Moines woman’s body was found in the trunk of the car.’ Then the car flashed across the screen,” Twyla told Dateline. “I didn’t think it was her. Then a police officer knocked on my door and passed me a funeral home’s card.”

Twyla told Dateline that she and her brother, Kenneth, went to the funeral home and identified Diane by her rings and watch. According to the local paper, the Des Moines Chief of Police at the time, Wendell E. Nichols, said that robbery did not appear to be a motive because the rings and watch would have likely been taken.

Twyla said she was told by authorities that the fabric used to strangle Diane was tested for DNA, but no matches were found, and no identifiable fingerprints were left at the scene. Dateline reached out multiple times to the Des Moines Police Department for comment on this report, but did not hear back.

Twyla told Dateline that she has put up a sign in her front yard with Diane’s name and information on it. She said, “You can't mention Diane’s name without everyone in this town knowing what you’re talking about. We pass out flyers every Sunday. We hold vigils all the time. People leave flowers, they honk -- so many people give their support.”

“I don’t need a whole army, I just need one person to help us,” Twyla added. “I have worked every single day trying to find answers.”

Twyla told Dateline she has just one message for her sister’s killer: “The night you killed my sister, you must have thought no one cares. You couldn’t have been more wrong. I care. I still care, and I'll care forever.”

If you have any information on the circumstances surrounding Diane Schofield’s murder, please call the Des Moines Police Department at 515-283-4864.