Im fairly certain that the knowledge necessary to build a nuclear bomb is far less well-protected than the general public is led to believe. The only part that is actually difficult is obtaining a sufficient quantity of enriched uranium without somebody dropping a JDAM on your lab.

If the Manhattan project could build a fat man in 4 years your state university's physics department has somebody who could reproduce that given the necessary resources. It might take them longer than four years given that they are not staffed with the modern equivalents of Oppenheimer, teller, et al but they could do it within a human lifetime if they actually wanted to cause a mass-casualty event and weren't afraid of the aforementioned JDAM.