Harker was last seen in Sioux City, Iowa on October 5, 1996. He resided in the 500 block of Colon Street at the time. He was supposed to meet his mother that day, but never showed up. She assumed he had forgotten. That same day, his white 1977 Chevrolet Suburban was found abandoned on a gravel road near Sioux City. Its doors were open, the fuel tank was empty and the tires were flat.

On October 9, after several attempts to contact him failed, Harker's mother went to his house and found it deserted. Harker's dog was there had not been fed, and he had left behind his medication. His mother became alarmed and reported her son's disappearance to police. She stated Harker was very close to his dog and took it with him most places, and it would be uncharacteristic of him to abandon the animal without making sure it would be cared for.

A local mechanic and friend of Harker's disappeared at about the same time Harker did. He returned home several days after Harker was reported missing, but moved away from the area shortly afterwards. The mechanic had a reputation for swindling people and he had many enemies as a result. Authorities searched the man's sister's property for Harker's remains, but found nothing. The mechanic was never charged in Harker's case.

Although there is no hard evidence of foul play in Harker's disappearance, investigators believe he may have been murdered. He enjoyed working with motorcycles and playing the guitar in 1996. His case remains unsolved.