Knockel was last seen in Dubuque, Iowa on November 26, 1990. He was driving a maroon two-door 1981 Mercury Sable or Mercury Zephyr with the Iowa license plate number NBL923 at the time of his disappearance. He was en route to celebrate Thanksgiving with his family, but he never arrived there, which is uncharacteristic of him. He has never been heard from again.

Knockel's family became concerned when he didn't show up for the holiday meal and they went to his house. He lived alone, worked menial jobs and rarely went out at the time of his disappearance. His loved ones found his wallet, watch and shoes at his home, but there was no sign of him or his car. The next day, his family thought they saw Knockel's car parked along Route 151, just over the Wisconsin border. A few hours later the vehicle had disappeared.

Knockel's loved ones believe he was abducted and murdered, as they think he would never have left of his own accord. Police stated he wasn't involved with drugs, alcohol or any high-risk activities and he didn't have any enemies at the time of his disappearance. Authorities think it's possible he went into the Mississippi River, but they're not sure whether his disappearance was accidental, a suicide or a homicide. His case remains unsolved.