Kim Byers will forever remember the unsettling contractor who entered her workplace and later took the life of her childhood friend.

On December 11, 1978, John Wayne Gacy arrived at Nisson Pharmacy in Des Plaines, Illinois, under the guise of assessing the store for a potential renovation. Robert Piest, a high school student working part-time, was assisting Byers, who was not only his coworker but also a friend and classmate. That day marked the last time Byers would see Piest alive.

Gacy was eventually found guilty of murdering 33 young men and boys in the Chicago region during the 1970s. Law enforcement uncovered most of the victims’ remains hidden in a crawl space beneath Gacy’s residence, with others buried elsewhere on the property.

An arrest photo of American serial killer John Wayne Gacy taken on December 21, 1978, at the Des Plaines Police Department. (Credit: Des Plaines Police Department/Chicago Tribune/Tribune News Service via Getty Images)

His gruesome acts have inspired a multitude of documentaries, books, and television series, including the recent Peacock series, “Devil in Disguise: John Wayne Gacy.” Byers’ daughter, Courtney Lund O’Neil, authored a book titled “Postmortem,” which delves into how Byers provided crucial information that eventually led to Gacy’s arrest.

“She bumped into John Wayne Gacy the night he took Rob,” O’Neil told Fox News Digital. “He wasn’t a nice guy. It almost seemed like he was there with a plan when you reflect on it — and she wasn’t of interest to him. But I do think he was watching Rob, and that’s why he came back later that night.”

Michael Chernus as John Wayne Gacy in the Peacock scripted series “Devil in Disguise.” (Brooke Palmer/Peacock via Getty Images)

In the book, Gacy is described as “a large man, overweight, his slightly receding brown hair laced with silver.” He refused to acknowledge Byers. When they bumped into each other in the pharmacy aisles, she looked into his eyes — a “dark omen indigo.”

WATCH: JOHN WAYNE GACY’S LAWYER EXPLAINS WHY SHE BELIEVES THERE ARE MORE VICTIMS

That chilly evening, Byers asked Piest, 15, if she could borrow his blue parka. While wearing it, she filled out a form to develop a roll of film and slipped it into the coat pocket. At the end of his shift, Piest asked for his jacket back to take out the trash. The contractor then returned to retrieve his forgotten appointment book.

He “lingered, noticing Rob,” O’Neil wrote. Gacy kept watching Piest as he pretended to browse the shelves. The killer often hired high school students for projects and took note of Piest’s work ethic and build.

“Postmortem: What Survives the John Wayne Gacy Murders” by Courtney Lund O’Neil is available now. (Citadel)

Piest was eager to get home — it was his mother’s birthday. But the contractor offered him a summer job paying $5 an hour, double the $2.50 minimum wage at the time. Piest was intrigued.

He assured his mother he would be home in time to celebrate. She was anxious to blow out her candles with the family. Piest left Nisson Pharmacy — and never returned.

Kim Byers previously spoke out in the Netflix true-crime documentary, “Conversations with a Killer: The John Wayne Gacy Tapes.” It’s currently available for streaming. (Netflix)

“Rob was taken on his mother’s birthday,” said O’Neil. “I always thought how horrifying that was for her — being a mother — and for the whole family. Rob loved his family and was excited to celebrate with his mom that night. And this one man destroyed everything. My mom saw her friend Rob leave with Gacy that night, and it changed her life.”

It wasn’t until the next day that an officer visited Byers and told her Piest never came home.

John Wayne Gacy’s original artwork of “Pogo the Clown.” (Steve Eichner/WireImage/Getty Images)

“This was a boy who went on nature walks and bought soda pop,” O’Neil said. “He was on his way to becoming an Eagle Scout. He was a family guy who loved gymnastics, canoeing and school. Rob was active and well-liked — the youngest in his family, full of dreams. He wanted to buy a Jeep for his first car when he turned 16 that spring. He was just a really good friend.”

Robert “Rob” Piest was 15 when he was murdered in 1978. Piest is seen here in 1978 before his death. (The collections of the Des Plaines History Center)

When investigators searched Gacy’s home, they found disturbing oddities — clown paintings, a hallway painted in bizarre yellow-and-brown zigzags, and a trap door leading to a crawl space. They also discovered startling mementos: a Maine West High School class ring, a pistol, handcuffs, keys and an ID that wasn’t Gacy’s. A film receipt from Nisson Pharmacy turned up among the evidence.

Police called the Byers home and asked the teen to come in for a statement. Byers later wrote about the roll of film she developed.

A blue jacket belonging to Maine West sophomore Robert Piest was found in John Wayne Gacy’s home. (Erin Hooley/Chicago Tribune/Tribune News Service via Getty Images)

O’Neil said there was never any question her mother would cooperate with authorities, hoping her memories might help solve what happened to her friend.

Courtney Lund O’Neil, a California-based writer, said Rob Piest’s murder forever impacted her mother’s life. (Leah and Tawn Nguyen)

“This was very scary to do at this time,” she said. “Being a young woman, her gender and her age were often used against her by authority figures. It wasn’t a time when a 17-year-old’s voice held a lot of weight.”

Byers’ receipt became a key piece of evidence during the investigation of Gacy, People magazine reported. According to the outlet, Gacy denied to police that he’d ever met Piest. But the police, who had Byers’ receipt in their possession, knew he was lying.

John Wayne Gacy, 36, a convicted sex offender, covers his face as he is led to a courtroom by a police officer. He was charged with the murder of a young boy. Investigators uncovered he had killed more and buried their bodies under his home. (Getty Images)

Police increased surveillance on Gacy. Inconsistencies in his statements, along with new evidence, led to a full search of his home. That’s when authorities uncovered human remains in the crawl space. Gacy eventually confessed to killing dozens of young men and boys.

The backyard structures of John Wayne Gacy’s home, circa 1979.  (Walter Kale/Chicago Tribune/Tribune News Service via Getty Images)

Piest’s blue parka was also found.

Though Gacy is frequently mythologized as a calculating serial killer, O’Neil countered that perception, pointing out he was “quite boring.”

Teenage boys and young men whose bodies have been definitely identified as the victims of American serial killer John Wayne Gacy. Top row, left to right: Sam Dodd (Stapleton), Robert Winch, James Mazzara, Richard Johnston and Jon Butkovich. Bottom row, left to right: Matthew Bowman, Robert Gilroy, John Szyc, Robert Piest and Frank Landingin. (Getty Images)

“I believe he managed a KFC. So what?” she explained. “He had his own construction business. Well, a lot of people do that. He was interested in politics. So are many other people. He had a picture with the first lady. Many people pose with celebrities. None of this is interesting on its own. 

Undated file photo of American serial killer John Wayne Gacy, circa 1975. (Getty Images)

“Even when they later studied his brain, there was nothing interesting to be found. It was quite a bland, boring brain. So I remind people in my book not to put serial killers on pedestals, but to actually look at them for who they are. And a lot of things about serial killers get amplified in a way that can be dangerous. It makes people obsessed.”

GET REAL-TIME UPDATES DIRECTLY ON THE TRUE CRIME HUB

Francis Wayne Alexander’s remains were discovered in December 1978. However, he wasn’t identified until October 2021 thanks to modern DNA technology. (Cook County Sheriff’s Office/AP Images)

“I think he was very clearly a master manipulator,” she continued. “He had charisma, and he could talk himself into and out of anything. And he had done that for the past murders before Rob Piest. With today’s technology, Gacy couldn’t do any of this successfully. But the fact that he was a leader in his community, charismatic and manipulative, made it a perfect storm for him to hide in plain sight for so long.”

A barren plot of land is an uncomfortable reminder of the “house of horrors” home of John Wayne Gacy that once stood there. (Bettmann Archive/Getty Images)

As crews dug through Gacy’s property, they uncovered bodies stacked beneath the home. Piest’s body wasn’t among them. Gacy admitted dumping other victims in the Des Plaines River after running out of space.

Police divers later recovered several remains from the river. Piest, who is believed to be Gacy’s final and youngest known victim, was identified through dental records.

This undated photo shows remains and evidence being removed from John Wayne Gacy’s home. (Getty Images)

Between 1972 and 1978, Gacy worked as “Pogo the Clown” at children’s parties. He also lured his victims to his home, where he murdered them. In 1980, Gacy was convicted of killing 33 young men and boys, making him one of the most prolific serial killers in U.S. history.

Sheriff Thomas J. Dart announced the identity of one of John Wayne Gacy’s unknown victims during a news conference at the Daley Center in Chicago on Nov. 29, 2011. The victim, William George Bundy, who went missing in October 1976, was 18. Bundy had been working construction jobs for Gacy. (Cook County Sheriff/Chicago Tribune/Tribune News Service via Getty Images)

“Even when they went to trial in 1980, my mother tried to make eye contact with Gacy when she took the stand,” said O’Neil. “He wouldn’t look at her. He told authorities when the investigation began that she was just a teen girl who wasn’t telling the truth. He insisted he never talked to a guy named Rob who worked at the pharmacy with her. But she knew.”

O’Neil said the case haunted her mother for years. She often wondered — if that film receipt hadn’t been found, how long would it have taken to stop Gacy?

In 1995, Gacy was executed by lethal injection at age 52.

Sheriff Tom Dart and Detective Sergeant Jason Moran announced on July 19, 2017, in Chicago, the identification of one more victim of John Wayne Gacy, James Byron Haakenson of Minnesota. (Nancy Stone/Chicago Tribune/Tribune News Service via Getty Images)

“My mom always remembered Rob’s love for his family,” said O’Neil. “He loved his family more than anything. He was just a kid who had so much life ahead of him. It was all just taken away.”

“As a little girl, my mom would always tell me to be careful about men who wanted to speak with me while I was riding my bicycle,” she reflected. “We were always on alert. I always felt something had happened. It wasn’t until I got older that I learned what happened, how she lost her friend one night.”