Robert Piest couldn’t be a runaway, thought Joe Kozenczak. The 15-year-old vanished from outside the Des Plaines pharmacy where he worked while his mother was just feet away, inside Nisson’s drugstore, waiting to drive her son home so they could cut into her birthday cake.

“That was a big red flag to Joe,” then chief of detectives for Des Plaines, said his wife, Karen. “This was a loving mother picking up her son, and he never returned.”

On December 11, 1978, Rob told his mother he was going outside to talk to a contractor about a summer job. Another employee said the man who wanted to see Rob was John Wayne Gacy.

Joseph Kozenczak in 1978. | Sun-Times library

Joseph Kozenczak in 1978. | Sun-Times library

Mr. Kozenczak’s investigators started looking closely at Gacy, a Democratic precinct captain for Norwood Township. Months earlier, he had been photographed shaking hands with first lady Rosalynn Carter. Sometimes, he entertained small children dressed as “Pogo the Clown.”

The investigators discovered that Gacy had served time in Iowa for a sex act with an underage boy.

Mr. Kozenczak, 75, who died of an infection May 13 at Advocate Lutheran General Hospital, was galvanized by the disappearance of Rob, who was in the same class as his son at Maine West High School. “As I read the report I thought to myself — this could have been my son,” he wrote in his book on the case, “The Chicago Killer.”

Then, a break in the case occurred. Rob Piest’s mother told Mr. Kozenczak that the jacket Rob was wearing had a receipt in the pocket from Nisson’s for developing a roll of film — something a friend left behind when she borrowed the jacket.

Mr. Kozenczak wrote of the electrifying moment in his book: “ ‘Wait one minute,’ I suddenly said, and, jumping up, bolted down the hall to the polygraph room. There, in a clear plastic bag, shining like a piece of gold, was the bright red receipt I had plucked out of John Gacy’s garbage bag. . . . This was the first and only piece of evidence we had showing that the Piest’s boy had even been in the house on Summerdale. I felt as if I had just dropped a noose around the burly contractor’s neck.”

Des Plaines detectives started a 24-hour surveillance on Gacy. At first the contractor joked with the investigators and invited them to breakfast. But then he started to crack. He referred to the chief of detectives as “S——- Kozenczak.”

“In five days, under the surveillance, his behavior crumbled to the point that he stopped shaving and started drinking and using drugs and shouting at people,” said Robert Ressler, a criminal profiler with the FBI who advised Mr. Kozenczak on the Gacy probe.

Gacy later confessed to multiple sex killings of young men and boys at his home. Typically, he handcuffed and strangled them. Some, he picked up on the street. He lured others under the pretext of offering jobs.

In 1980, he was convicted of more murders than anyone in U.S. history. Fourteen years later, he was executed for the slayings of Robert Piest and 32 others, most of whom he buried in the crawlspace of his home at 8213 W. Summerdale.

Joe Kozenczak and his wife, Karen, co-authored "The Chicago Killer," a book on the John Wayne Gacy murders. | File photo

Joe Kozenczak and his wife, Karen, co-authored “The Chicago Killer,” a book on the John Wayne Gacy murders. | File photo

Mr. Kozenczak grew up in Humboldt Park. He attended Weber High School before his parents moved to River Grove, where he graduated from East Leyden High School. He turned down a football scholarship to Colgate University to become a military police officer in the army in the late 1950s, his wife said.

Mr. Kozenczak served as chief of police of Des Plaines from 1985 to 1989.

He and his wife co-wrote “The Chicago Killer.” His intuition helped crack the case, she said. “He could almost tell you what you were thinking, when you were thinking it.”

“He was a good man,” said retired Des Plaines police chief Lee Alfano. “When he got onto something, he was like a bulldog. He stayed right with it till it was done.”

In the forward to Mr. Kozenczak’s book, Ressler wrote, “There will never be another serial murder case of the magnitude of the case of John Wayne Gacy, nor will there ever be another friend, colleague or dedicated law enforcement officer like Joe Kozenczak.”

Mr. Kozenczak was a technical adviser on the 1992 TV movie “To Catch a Killer,’ starring Brian Dennehy as Gacy.

He loved Sherlock Holmes mysteries, especially the PBS shows featuring Jeremy Brett. He enjoyed watching all three of “The Godfather” movies in sequence, and the film “Fiddler on the Roof.” His Russian Blue cat, Jade, “misses him terribly,” his wife said.

After retiring, he worked as a private investigator. He also studied at the Evanston Art Center and displayed his paintings and etchings at shows, his wife said.

Mr. Kozenczak also is survived by their daughter, Natalie Jude Hua Kozenczak, and his children with his former wife, Marcia: daughters Lisa Marie Kozenczak and Deborah Ann Zerfas; a son, Michael Joseph, and two grandchildren.

A memorial gathering is to begin at 9 a.m. Friday until a 10 a.m. Mass at the Shrine of Our Lady of Guadalupe, 1170 N. River Rd., Des Plaines.