Bennett said he didn’t know whether North Korea could place a nuclear warhead on a missile, but that South Korean defense experts think it’s possible. “Many of my South Korean colleagues argue they can put a warhead on a missile and may have done so already,” Bennett said. “I don’t know for sure, but my guessing based on what my South Korean colleagues are telling me is that they can.”

Inside the U.S. intelligence community, the issue of North Korea’s progress is hotly debated. One problem is that while U.S. spy satellites can monitor the country from overhead, most of North Korea’s nuclear work is done underground. The country is so closed off that the Central Intelligence Agency has also had trouble recruiting high-level spies inside the country.

One piece of evidence to support the view that North Korea can produce a nuclear warhead comes from A.Q. Khan, the man considered to be the father of the Pakistani nuclear program. In correspondences with a former British journalist, Simon Henderson, first disclosed by the Washington Post in 2009, Khan said that during a visit to a North Korean nuclear facility in 1999, he was shown boxes of components for three finished nuclear warheads that could be assembled within an hour.