Physical Evidence:
* Autopsy: The autopsies of the victims were crucial for determining the cause of death, time of death, and identifying any injuries or signs of struggle.
* Odontology: Dental records were used to identify some of the victims.
* Anthropology: The remains of some victims were fragmented, and anthropologists helped with identification and reconstructing the victims' profiles.
* Trace Evidence: Fibers, hairs, and other trace evidence were analyzed to link Gacy to the crime scenes and the victims.
* Ballistics: The bullets recovered from the victims were examined to determine if they matched Gacy's gun.
* Fingerprint Analysis: Fingerprints were found at the crime scene and on the victims, which helped to connect Gacy to the murders.
* Soil and Plant Analysis: Soil and plant samples found on the victims' clothing and in Gacy's car were compared to samples from the crime scene.
* Bloodstain Pattern Analysis: Bloodstain patterns found at the crime scene helped to reconstruct the events of the murders.
Other Techniques:
* Polygraph Tests: Gacy was subjected to polygraph tests during the investigation. While not considered conclusive evidence, they helped to raise suspicion and guide investigators.
* Psychological Profiling: Investigators used psychological profiling to understand Gacy's motivations and predict his behavior.
* Witness Interviews: Witnesses who saw suspicious activity around Gacy's home or who knew Gacy personally provided valuable information to the investigation.
The use of forensic science in the John Gacy case was groundbreaking for its time and helped to convict him of the 33 murders. The investigation also highlighted the importance of using a multidisciplinary approach to forensic science, incorporating the expertise of various specialists to obtain a comprehensive picture of the crime.