Defences currently in place against a nuclear attack are far more effective than US policymakers may believe, according to an expert on weapons of mass destruction and science and technology in the Islamic world.
Michael Levi, author of On Nuclear Terrorism and fellow at the Council on Foreign Relations, suggested that there is no perfect defence against nuclear attacks, but chances of an attack are lower than some may suggest.
"In some sense what I'm trying to do in this book is help kick the habit of falling into those different traps."
Levi said defence should be accounted for as a whole, instead of each piece separately. Often Congressional inquires simply test one part of a defence, ignoring its implications on the rest of the system, he said.
"Yes, it is useful to do audit and tests of little pieces of the defence as long as you make sure you can learn from that, but there is a tendency to focus on any one piece and show how you can defeat it," said Levi.
"You also have to ask 'what would the implications of that be towards vulnerability to the rest of the defence?', 'what opportunities does that create for new approaches to defence?'"
He added the perception of how terrorist groups function is not grounded in reality. The most capable groups are also most likely to fail, and that presents an opportunity to fend off attacks.
"After 9-11, we got into this mentality where we said people aren't afraid of dying, that means deterrence is dead," Levi said. "Well, against certain people and certain plots, the threat of retaliation isn't nearly as useful as it once was, but the prospect of failure can be a lot more powerful especially against leaders of these groups."
Terrorists want to avoid failure to avoid being judged as a failure in the eyes of God and the community, Levi said. There is also pressure to not fail in order to recruit members and build up authority.
"A group that constantly tries things and fails spectacularly has difficulty building up the kind of capability that would be required for a sophisticated plot," he said, adding, "Then you really need to think of imperfect defences as more powerful than you otherwise think they are."
It is also difficult for terrorists to develop a plan using nuclear weapons because of the many different parts involved. It may be easy to acquire information or nuclear materials, but not so easy to build a bomb or cross borders, Levi said.
"When it comes to making nuclear weaponsÂ… engineering is harder," Levi said. "Yes it can be done, a group could have the right people, you do not need a Manhattan Project style, yet alone modern nuclear capabilities. But there are a lot of different ways that things can go wrong."
One such example he cited was the Iraqi effort to cast uranium that was unable to remove all of the impurities. A bomb created using such material would have a yield hundred or thousand times lower, resulting in a smaller explosion and thus fewer casualties.
"Still a horrible day, but still a difference," he said. "The difference between a hundred thousand people killed and five thousand people killed is a real difference."
With a combination of skills required, he added, a terrorist group is unlikely to possess all of them.
"People will say 'what happens if a group of Pakistani scientists and engineers in the physics lounge decide they are very upset at America and they are going to pull off a nuclear attack?'
Well, that's something we should be concerned about, but we still should ask where does that group not only have the capabilities to get materials and move things around the world, but what are their connections to start to get the right people to go in that direction. Maybe they could find them, maybe they would run into difficulties," Levi said.
Levi did not go as far as to make policy recommendations, but did say that working with countries to shore up their nuclear arsenals is better than threatening them.
"Threatening countries to hold them accountable for not being sufficiently vigilant in controlling their materials is an extraordinarily counterproductive approach to things," he said.
He also said that assessments of defences should include experts from nuclear and terrorism sides, something that is happening more often, but not as much as it should. Another recommendation included sharing sensitive information, not only with local authorities and international allies, but also with enemies in order to practice effective deterrence.
Levi called himself a "pragmatic agnostic" and encouraged other people to take the viewpoint as well. He distinguished himself from "nuclear zealots," who believe an attack is certain, and "nuclear atheists," who believe the chance of an attack are close to zero.
He purposefully avoided putting a number of the likelihood of an attack, and emphasized the need to look for ways to improve defences.
"There is a bit of a fear out there in a lot of the policy community that think that if you make an argument that suggests that nuclear terrorism maybe be less likely than you think or harder than you think, that somehow it will lead us to do less to prevent it," Levi said.
"There is a real unfortunate possibility that if you overestimate the ease of pulling off an attack, you will do less to prevent it."