A friend of mine were discussing the question of torture. Specifically, given the context of the last few months, would it be ethical to torture a person if it were known that this person had knowledge of a nuclear bomb in a large city. We don't know which city or when it will happen, but we suspect he has the knowledge. We both agreed that the ethical questions were difficult. I am writing to you if you know of any Talmudic web resources that can answer this question. I have been searching for 2 days and have not found one.

This is indeed a difficult question.

Certainly the accusations that this person has the knowledge has to be very well based; not just a conjecture. Once authorities decide that there is hard evidence that this person does have such information, it is probably a duty to apply reasonable physical pressure on this person to reveal what he knows, in order to save lives. (source: "Maimonides" Laws of Murder 2:4-5; based on Talmud Sanhedrin 46a)