Is torture justified for national security?
First, it is illegal.
Second, would it be justified for others to torture Americans for their national security? These questions are easier to answer if you recognize the moral principle of universality. If it is reasonable for us, it is reasonable for them. Is it justified to torture a DIA agent to get information about a drone strike in Yemen? Will that be helpful in the long-run?
Third, it doesn't work. If you look at the documents that have been released about the CIA torture program, you find that very little intelligence was gained. In fact, none that could not have been obtained by other means.
And, finally, the way to keep the country safe from "terrorists" has been known for many years - treat people with justice so they do not become terrorists. If no serious effort is made to do this, then perhaps those recommending torture are not interested in national security, and we would be wise to inquire into their actual goals.
It is a contentious issue. Can we deny civil liberties to a few people in order to benefit many? Do all humans have equal rights and liberties?
There are pros and cons to the argument. However, I would go with the theme of "for the greater good." Consider how many times a human life is saved by amputating a festering organ/limb. In the same vein, "festering" components of society may be removed if the overall survival of society is at stake. In a recent survey, 59% of Americans supported the CIA's interrogation program. It would be interesting to see if the responses are the same for the interrogation of foreign nationals vs. that of Americans.
Consider another scenario: a suspect may hold information crucial to locating a bomb or assassination plot. With the clock ticking, we may have to do what we must to ensure that the necessary information is extracted.
There are those who may argue that every human life is worth the same amount and occasionally innocent people may be the suspects, but considering the big picture, interrogation and torture may be an effective tool to save many.