Question

Who could possibly have believed him to be guilty? / Yet today he is behind the bars in Alcatraz.

The first member of the pair is a fragment, and the second is complete
The second is a fragment, and the first is complete.
Both are complete.
Both are fragments.

Answer 1

Who could possibly have believed him to be guilty ? -- independent
Yet, today he is behind bars in Alcatraz -- independent

Both are complete

Answer 2

Both are sentences are complete.