Ever since he was a kid, Jeff Haakenson wanted to know what happened to his uncle who disappeared in Chicago without a trace.

Haakenson was born in 1980 and never knew his dad’s brother, but about 20 years ago he began channeling his curiosity into a keyboard and became an online sleuth.

All he had to work with was a name — James “Jimmie” Haakenson — and a story that went like this: In 1976, Jimmie, one of four siblings, left his family home in the Minneapolis/St.Paul area when he was 16 to have an adventure in Chicago. Apart from a phone call shortly after he left reporting to his mother that he was OK, no one ever heard from him again.

The fact that his uncle just up and left at such a young age, seemed bizarre to Jeff Haakenson, who lives in Lubbock, Texas.

“But I talked to a lot of people and they said that was kind of normal back then,” he told the Chicago Sun-Times Thursday. “He would leave and a couple days later he would come back, but he always called home to tell his mother he was OK.”

Related: EDITORIAL: Dart delves into darkest Chicago past to solve Gacy mysteries

Haakenson learned from relatives that after initial efforts, the search languished.

“That bugged me,” he said. “It was like he just vanished and it just drove me nuts. How could somebody just disappear off the face of the earth?

“He was like a ghost.”

There was speculation in the family that Jimmie had fallen victim to serial killer John Wayne Gacy, who was known to have been active when Jimmie went missing. But Jeff Haakenson took this more as rumor than theory.

“I would always say, what are the chances that he ran into Gacy? Chicago is such a big city.”

A sheriff’s poster identifying James “Jimmie” Haakenson | Mitch Dudek/Sun-Times

Haakenson started his online search when he was 19 and serving in the Air Force. He’d lounge in the barracks and surf the web on a laptop.

Over the years he’s left messages on online forums asking for help, paid an online ancestry service to join the search, sifted through missing-persons databases and spent hours playing with different combinations of his uncle’s name.

More recently, Haakenson, 37, surfed the web on his daughter’s desktop. Sometimes the 13-year-old would kick her dad out of her room.

One day last summer, during his lunch break, Haakenson sat down at a work computer in an aircraft hangar in Los Angeles, where he worked at the time as a mechanic, to run yet another search. He decided to look into the Gacy connection.

He hadn’t directly broached the subject with his dad’s other siblings because he thought it would be awkward. “I don’t know why that day I decided to Google John Wayne Gacy victims, but all of the sudden, bam,” Haakenson said.

Up popped a page on Cook County Sheriff Tom Dart’s website seeking the public’s help in identifying the remains of seven Gacy victims.

Investigators had identified 26 other bodies since Gacy was caught in 1978, most of which were found buried in a crawl space beneath Gacy’s ranch-style house in the northwest suburbs.

Haakenson filled out an online form and a few months later got ahold of Sheriff’s Detective Jason Moran.

Haakenson’s father, Donald, sent in a DNA sample that matched part of the genetic makeup of the body. The missing teen’s sister, Lori Sisterman, was then asked for a DNA sample,  and it proved conclusive.

Detective Moran met with family members in Minnesota on Tuesday to tell them what happened to Jimmie. As relatives shared memories, more details came to light.

The missing teen’s mother, June, traveled to Chicago shortly after he disappeared with Lori, who was 20 at the time. The pair went around town showing people his photo asking if anyone had seen him.

When news later broke about Gacy, June shared her concern that her son might have been killed by Gacy with the St. Paul Police Department. The St. Paul cops reached out to Gacy investigators, but the family had no dental records for the teen — and that’s how bodies were identified back then. June died in 2008 never knowing her son’s fate.

“I forgot about (Gacy),” Sisterman said Thursday. “We never thought of that in all those years.”

When Sisterman thought of her brother over the years, she thought “maybe he’s rich and the CEO of a company sitting in Tijuana drinking Mai Tais. Or maybe fell off a cliff and is sitting at the bottom. But if he’s rich, why isn’t he getting a hold of us.

“I imagined good things and the worst. But I never imagined that man,” she said of Gacy.

“This brings closure,” Jeff Haakenson said. “I’m glad, in a way, we found it. The ending is horrible, to know that Jimmie had that happen to him. That should have never happened.”