On June 21, 1978, 22-year-old Carla Brown was found brutally murdered in the basement of her home.

It looked as if the killer had committed the perfect crime.

And police could find very little evidence.

But years later, investigators noticed something on a crime scene photograph

that had previously been overlooked.

The killer had left behind a clue.

22-year-old Carla Brown and her fiancé, a man we'll call Mark Hart, had recently purchased a home together in Wood River, Illinois.

Although the couple had a rocky past, friends and family saw this as the beginning of a new phase in their relationship.

Carla Brown was a beautiful young woman.

She was very popular in her hometown in the Wood River area.

She was one of those girls that everybody knew.

Girls liked her, guys liked her. She was just one of those people that was special.

And everybody recognized that.

On June 21, Carla Brown took the day off from work to unpack.

Yes, the house is bright.

And spoke to one of her friends on the telephone.

Maybe we can have you ever for dinner.

They had talked for a few minutes. In fact, we were even talking about getting together later on that afternoon.

Why does it take a long time to start something with the door?

And Carla interrupted the conversation and said, let's not have to call you back. I'll see you later. There's somebody at the door.

And that was the last that her friend ever talked to.

Around 5.30 that afternoon, Carla's fiance and his friends stopped by the house after work.

...