Robert Piest and John Wayne Gacy.Photo:Des Plaines History Center, Des Plaines Police Department/Chicago Tribune/Tribune News Service/Getty

Robert Piest and John Wayne Gacy

Des Plaines History Center, Des Plaines Police Department/Chicago Tribune/Tribune News Service/Getty

On Dec. 11, 1978, serial killerJohn Wayne Gacywalked into Nisson Pharmacy in Des Plaines, Ill., to discuss a remodeling job with the store owner. Little did anyone know at the time, he was about to encounter his final victim — and evidence that would help put him behind bars.

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On December 11, 1978, Kim arrived late at Nisson Pharmacy for her 5 o’clock shift. It was 31 degrees when she arrived. Rob came to work shortly after her, wearing brown pants, a beige shirt and his favorite blue parka. She worked the register, and Rob busied himself pricing and checking in new items to be shelved.A new person was working in the store that night. He was a large man, overweight, his slightly receding brown hair laced with silver. The man did not acknowledge Kim. They bumped into each other in the aisles of the pharmacy.“Who is that guy?” Kim asked the owner, Phil, at the pharmacy counter.“A contractor,” Phil said. “Larry and I asked him to take measurements for the store. You know, a possible facelift.” Phil said it like the contractor was a family friend.

The rest of Kim’s shift dragged on. She cast around, ringing up guests and listening to conversations her boss had with the strange man in the store. He estimated the cost of the remodel would be $1,600. The contractor left around 7:15. He also left his coffee-colored appointment book on the pharmacy counter.

Bodies being removed from John Wayne Gacy’s house in December 1978.Karen Engstrom / Chicago Tribune/TCA

Bodies being removed from John Wayne Gacy’s house in December 1978

Karen Engstrom / Chicago Tribune/TCA

Around 8 p.m., it was nearing the end of Rob’s shift and he needed to take out the trash. “Hey, mind if I take my jacket back?” he asked Kim.Kim slid out of the blue parka and handed it to Rob. He zipped it up to his chin to keep out the cold.As Rob carried the plastic bags to the dumpster in the back, he noticed school-age kids playing in the snowbanks. A girl noticed him and flirtatiously chucked a snowball at him.“Hey!” Rob said. The girl laughed and ran down the alley, back into the dance at the Iroquois Junior High School. In the wind and snow, Rob hefted the trash over his shoulder and into the can [then] headed back inside.

Sometime in the last hour of Rob’s shift, the contractor returned. He grabbed his forgotten appointment book. He lingered, noticing Rob. He often hired high schoolers with good work habits to help him on various projects. It wouldn’t elicit suspicion if he offered this kid a job. The contractor meandered; irregularly walked the pharmacy aisles, pretended to eye the shelving again.

At 8:55 p.m. [Robert’s mom] Elizabeth Piest entered the store. She greeted Kim, she greeted Rob. The night was supposed to be special. Elizabeth was turning 46 and anxious to get back and blow out candles. She had a birthday cake waiting at home. The whole family was waiting for her and Rob at the house.Rob was finishing his stocking job while his mom waited and he approached Kim around 9 p.m. a final time. “That contractor wants to talk to me about a summer job that will pay me $5 an hour. Mind watching the front for a minute?” The minimum wage was $2.50, so the contractor’s offer made him light up at the possibilities the money could offer.Kim looked at Rob, and his image imprinted in her mind. Loyal, handsome, strong.“Okay,” she said. “See you soon.”He looked from Kim to his mother and said, “I’ll be right back.”“Okay, honey,” Mrs. Piest said. “I’ll be right here.”The door shut behind him, and she thought nothing of it. But for the rest of her life, she’d see his departure, the closing of the door in slow motion.

Robert Piest’s parents, Elizabeth and Harold Piest, arrive for his funeral on April 18, 1979.Karen Engstrom / Chicago Tribune/TCA

Robert Piest’s parents, Elizabeth and Harold Piest, arrive for his funeral on April 18, 1979.

Rob never came back into Nisson Pharmacy. After about 10 minutes, Kim went out back to look for him. He was gone. She saw footprints in the icy snow, tire tracks of the contractor’s car.Rob’s mother went out to look for him as well and saw nothing. But deep down, a rock immediately formed at the basin of Elizabeth Piest’s stomach. Where could he have gone?At 9:20 p.m., Elizabeth Piest had been waiting for over 20 minutes for Rob to come back inside the pharmacy. Kim told her that Rob asked her to watch the register at the front of the store, near the door, while he talked to the construction guy outside.

Never miss a story — sign up forPEOPLE’s free daily newsletterto stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories.Arriving back home, Elizabeth glanced at her birthday cake, suspended in time on the dining room table. “Anyone from Nisson’s call?” she asked her husband. Harold shook his head.She grabbed the telephone, hoping Rob had returned to Nisson Pharmacy and Kim forgot to call.Kim answered the phone. “Not yet,” she said. Mrs. Piest detected worry in the young girl’s voice.“Who was he talking to outside? Who was that man?”The family decided they would all go to the police for help.

source: people.com