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You can choose not to answer. If he will never find them then by not answering then you are neither lieing nor not betraying innocent people leading to their death.
EDIT For clarification of my position.

Taking no action would be preferable to taking an action against the Moral Imperitive.
Further clarification: Page 112 of this document shows that Kant clearly believed that lieing to a murderer that someone is not there is a lie. Even if the person was not actually there but the actor believed he was.
However, A lie of ommission is permissible. A declaration that is either known or believed to be false is not. How ever if the jews had been given refuge by the actors wife but the actor did not see them it would be permissible to say " I have not seen any jews this evening." Inviting a conclusion that there are no jews present. While not actually answering the question

You can choose not to answer. If he will never find them then by not answering then you are neither lying nor not betraying innocent people leading to their death.

Taking no action would be preferable to taking an action against the moral imperative. 

Page 112 of this document shows that Kant clearly believed that lying to a murderer that someone is not there is a lie. Even if the person was not actually there but the actor believed he was.

However, a lie of omission is permissible. A declaration that is either known or believed to be false is not. However, if the Jews had been given refuge by the actor's wife but the actor did not see them it would be permissible to say "I have not seen any Jews this evening," inviting a conclusion that there are no Jews present, while not actually answering the question

You can choose not to answer. If he will never find them then by not answering then you are neither lieing nor not betraying innocent people leading to their death.
EDIT For clarification of my position.

Taking no action would be preferable to taking an action against the Moral Imperitive.
Further clarification: Page 112 of this document shows that Kant clearly believed that lieing to a murderer that someone is not there is a lie. Even if the person was not actually there but the actor believed he was.

You can choose not to answer. If he will never find them then by not answering then you are neither lieing nor not betraying innocent people leading to their death.
EDIT For clarification of my position.

Taking no action would be preferable to taking an action against the Moral Imperitive.
Further clarification: Page 112 of this document shows that Kant clearly believed that lieing to a murderer that someone is not there is a lie. Even if the person was not actually there but the actor believed he was.
However, A lie of ommission is permissible. A declaration that is either known or believed to be false is not. How ever if the jews had been given refuge by the actors wife but the actor did not see them it would be permissible to say " I have not seen any jews this evening." Inviting a conclusion that there are no jews present. While not actually answering the question

You can choose not to answer. If he will never find them then by not answering then you are neither lieing nor not betraying innocent people leading to their death.
EDIT For clarification of my position.

Taking no action would be preferable to taking an action against the Moral Imperitive.

You can choose not to answer. If he will never find them then by not answering then you are neither lieing nor not betraying innocent people leading to their death.
EDIT For clarification of my position.

Taking no action would be preferable to taking an action against the Moral Imperitive.
Further clarification: Page 112 of this document shows that Kant clearly believed that lieing to a murderer that someone is not there is a lie. Even if the person was not actually there but the actor believed he was.