Knapp was last seen by his roommate at their residence in University Terrace Apartments at Clemson University in South Carolina on April 11, 1998. His roommate said that Knapp was watching a movie at approximately 10:30 p.m. that evening. Knapp has never been seen again. He may have been carrying an expired prescription card, a Keystone Health Plan Central HMO card, an MBNA America Bank card, a Wachovia automatic teller machine (ATM) card and an additional bank card at the time of his disappearance. After several days, Knapp's roommates became concerned because he had not returned to his apartment. They notified his mother, who reported him missing.

Knapp's white 1990 Chevrolet Beretta with the Pennsylvania license plate number APV9032 was discovered abandoned nine days after he was last seen on April 21, 1998. The vehicle was parked at Table Rock State Park in Pickens, South Carolina, approximately 30 miles from Clemson University, in a very rugged area. Authorities found a receipt from a local Wendy's restaurant dated April 12, 1998 at 1:30 p.m. inside the car. Investigators believe that Knapp drove to the park on April 12, but they are uncertain if he was alone. There was no sign of foul play at the vehicle's location. Authorities searched the surrounding terrain for two weeks and found nothing of interest.

Knapp withdrew $20 from his bank account on the day he disappeared. There hasn't been any activity on the account since then. He left behind most of his belongings, including his camping gear. He had planned to search for another apartment the weekend after his disappearance.

Knapp was an ROTC student at Clemson University at the time of his disappearance. He majored in mechanical engineering and was doing well academically. He is originally from Pennsylvania, and graduated from Central York High School in 1996. Knapp's mother said that her son was interested in working for a NASCAR racing team in the future. He enjoyed driving cars at fast speeds. Knapp also enjoys mountain bicycling and target shooting. He was previously employed as a restaurant busboy. He had pledged into the Pershing Rifles, an elite military organization, just before his disappearance. His case remains unsolved.