A Logic Puzzle

This is not my own, unfortunately. The late, great Raymond Smullyan may have created it.

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Boy -- Proposes Marriage to Girl.

Girl (with a titter, having already prepared): "Of these three boxes, one contains a dagger, while the other two are empty. I'll gladly marry you if pick and open an empty box."

Written on box one: "The dagger is in here."

Written on box two: "This box is empty."

Written on box three: "At most one of these three statements is true."

The boy thinks very carefully, then opens one of the boxes. He freaks out. "Ahhhh!"

Which box did the boy open, what did he find, and where did he go wrong?