A number of people wanted to, but the investigation failed to reveal they

The who that is used here is an interrogative pronoun, the only word in a reduced embedded interrogative clause. Similar to:

We wondered [who might have wanted to kill Robert].

Someone might have wanted to kill Robert. We wondered [who].

It cannot be a relative pronoun because present-day English does not allow fused relatives with a relativized subject using who.

*Who might have wanted to kill Robert might also have wanted to kill his dog.

Even if the above were a valid construction, the notion of reducing a relative clause down to just the relative word is absurd. The word would invariably be interpreted as a reduced interrogative where possible, and produce an ungrammatical result otherwise,

Mark gave me a gift. *I didn't like what.

Jenny chose a place for the party. *We're all going where tomorrow.

Bill chose a new partner. *I'm surprised at who.

with the exception of those with -ever suffixes.

You can go with whoever [you like].

Hence, who must be subject in the embedded interrogative, excluding whom. Of course, if the understood interrogative were changed to query an object, then whom would be possible.

Katy chose someone. She'll soon reveal whom.

A nominative pronoun could not be used because that would constitute a different construction - one without an embedded interrogative as complement to reveal - as there would be no interrogative word or phrase. The pronoun would simply figure as the object of reveal requiring the accusative case.