There was a crime, and it was known that either Al, Beth, Dell, or Phil (each a knight or a liar) was guilty. At the trial, the men spoke first, and said:

Phil: Dell did it.
Al: No, Beth is guilty.

Then it was the ladies' turn, who said:

Beth: Al is lying.
Dell: Phil never lies.

Then a lady accused someone, and that let me know she herself was innocent.

And next the other lady accused someone, and that let me know who was guilty.

Who's guilty?