Mr. Gacy was convicted in 1980 of killing 33 people and was executed in 1994. But identifying the remains of his victims has been a decades-long process aided in recent years by advances in forensic technology and the use of DNA evidence.
The police notified Mr. Haakenson’s family on Monday that his remains had been identified.
“It’s not every day you heard this: A monster murdered your brother,” his sister, Lorie Sisterman, told a CBS affiliate in Chicago. “It’s just not an everyday, normal conversation that you have with a detective from a different state who tells you this awful news.”
Mr. Haakenson’s family last heard from him on Aug. 5, 1976, when he called home and told his mother that he was in Chicago, the department said. Investigators believe he crossed paths with Mr. Gacy soon after that call.
Mr. Haakenson’s body was found in Mr. Gacy’s crawlspace in December 1978 in a makeshift grave alongside two others. One of those bodies was identified at the time as Rick Johnston, who was last seen on Aug. 6, 1976.
The department said it believed Mr. Gacy murdered Mr. Haakenson and Mr. Johnston either at or around the same time based on the last time they were seen and their relative positions in the crawlspace. Investigators have not been able to identify the third body in that grave, which is classified as Victim No. 26.
Thomas J. Dart, the Cook County sheriff, reopened the investigation to identify the remaining victims in 2011, when there were eight whose identities were not known. Since then, investigators have identified one other victim: William Bundy, a 19-year-old who disappeared in October 1976.
The department said Mr. Haakenson’s mother tried to determine in 1979 if her son’s body was among those found under Mr. Gacy’s house, but dental records, which were the primary means of identification at the time, were unavailable. She is no longer alive, the department said.
The authorities identified Mr. Haakenson’s remains by requesting DNA samples from two of his siblings and submitting them to the University of Northern Texas Center for Human Identification. They also reviewed the original missing person’s report filed by Mr. Haakenson’s family, information from the Social Security Administration and post-mortem reports.
Mr. Gacy’s unidentified victims were buried in 1981 under tombstones inscribed with the words “We Are Remembered.” The sheriff’s department said its investigation into the identities of the six remaining unknown victims was continuing.