An ancient book known as the Perplexicon was stolen from the Limitless Enigmatic Vault of Innovative Knowledge yesterday. When security made their rounds at 6:00pm, they noticed it was missing and went on lock-down. The museum's security tapes for 5:00pm-6:00pm yesterday are missing. Police pulled in suspects for questioning, and each claimed to have an alibi:

Charlie: "I was attending a convention."
Gamer: "I was at the movies."
brianjn: "I was bowling with my friends."

Since police could not determine who were knights, liars, or knaves, let alone who was the thief, they called in the renowned detective Dustin to question them. While the other investigators listened from the next room behind a two-way mirror, the following took place in the interrogation room:

Dustin: "On the night in question, Gamer, you were NOT at the movie theater. You were, in fact, stealing the Perplexicon!"
Charlie: "If Gamer is guilty, then I'm a Martian."
Dustin: "And you, Charlie, claimed you were at a convention."
Charlie: "That is true."
Dustin: "And yet, an eyewitness claims to have seen you fleeing LEVIK at 5:15pm!"
Charlie: "I already said I was at a convention."
Dustin: brianjn, you're being awfully quiet. Your alibi doesn't check out either."
brianjn: "Just because a person's alibi doesn't check out doesn't mean they are guilty. Besides, I really was bowling."
Charlie: In fact, brianjn invited me to join his bowling group, but I had to decline."
Dustin: "Hey, I'm the investigator! I'll ask the questions! Someone in this room is guilty."
Charlie: "It is not I."
brianjn: "It is not I."
Gamer: It is not I."

Who stole the Perplexicon?