Which is ambiguous, of course. This is produced because modal auxiliaries like must are very tightly constrained positionally in English, while the negative quantifier only can oprionally precede any constituent containing the negative focus, so it can wind up outside the scope of the modal, like this sentence does, producing the wrong effect..

To fix the construction, you have to make the syntax resemble the semantics, so one solution is that the only works better if it’s isolated with the agent at the end of the sentence — well inside the scope of the square modal — a situation that simply applies Passive. If you passivize this clause, you gotta keep the agent.