Court documents detail what a man charged with murder told investigators about the abduction and killing of Mollie Tibbetts.
I'm not sure what 'what' is doing in this sentence. 'Detail' here is used as a verb. "charged with murder" modifies "a man". "A man told investigators about the abduction and killing of Mollie Tibbetts" is the main body of the sub-clause. So, I think 'that' is just fit.
Court documents detail that a man charged with murder told investigators about the abduction and killing of Mollie Tibbetts.
Any thoughts?
The full source.