On a distant island live three types of humans - Knights, Knaves and Normals. The Knights always tell the truth, the Knaves always lie, and the Normals sometimes lie and sometimes tell the truth.

Detectives questioned three inhabitants of the island - Al, Bob, and Clark - as part of the investigation of a terrible crime. The investigators knew that one of the three committed the crime, but did not at first know which one. They also knew that the criminal was a Knight, and that the other two were not. How they knew these things is not important for the solution.

Additionally, the investigators made a transcript of the statements made by each of the three men. What follows is that transcript:

Al: I am innocent.

Bob: That is true.

Clark: Bob is not a Normal.

After carefully and logically analyzing their information, the investigators positively identified the guilty man. Was it Al, Bob or Clark?