doing all they can/should/must

[joham]

The police still haven't found the lost child, but they're doing all they ________ ( can/ should/ must).
The given answer is 'can'. Could we think of some special context where 'should' or 'must' can be used?

Thank you in advance.

[English Freak]

No, it makes no sense to use other modal verbs in this particular example. Not really meaningful.