In Macbeth, what should an good investigator ask Malcolm?

Malcolm was in the castle when his father King Duncan was
killed.  His brother Donalbain was also there, but Malcolm is the older brother.  In
fact, since Malcolm is next in line to be king, he would be the number one suspect. 
Unfortunately, Malcolm and Donalbain flee before anyone can ask them anything, making
them look guiltier than ever.

An investigator looking into
Duncan’s murder would naturally start with Malcolm.  The first question should be: Are
you next in line to be king?  The answer is yes!

The
investigator would likely remind Malcolm what Duncan
said.

Sons,
kinsmen, thanes,

And you whose places are the nearest,
know

We will establish our estate
upon

Our eldest, Malcolm, whom we name
hereafter

The Prince of Cumberland;
(1:4)

Yet a good investigator
would also ask who was present at this announcement, and how they reacted.  A person
might have noticed that Macbeth seemed disturbed.  He even mumbled to
himself.

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[Aside.] The Prince of
Cumberland! That is a step(55)

On which I must fall down,
or else o'erleap,

For in my way it lies. Stars, hide your
fires;

Let not light see my black and deep
desires(1:4)

So asking the
right question makes all the difference.  The investigator that asked this question
would know to look at Macbeth as a suspect.

Another
important question might be who else is in line for the throne after Malcolm.  The
answer seems more likely to be his brother Donalbain, because Donalbain also
flees.