Were Ward and Mrs.Collins in a conspiracy to kill Whitmore?
As if divining what was in their minds, Britz proceeded to answer their unspoken query.
"To attribute the crime to Mrs.Collins or Ward, or to both of them," the detective said, "it is first of all necessary to find a motive.

Only one suggests itself.

It is that they killed Whitmore to get possession of his estate.
"We must remember that had Whitmore died intestate, neither of them would have obtained a penny of his fortune.

So that, in order to establish our motive, it is necessary to prove that they had knowledge of the contents of the will.

All the evidence I have gathered tends to contradict that assumption.