Robert Carlin

A nuclear expert who toured North Korea's Yongbyon nuclear facility with Siegfried Hecker, the co-director of the Center for International Security and Cooperation at Stanford University, has described his shock at seeing the facility.

Robert Carlin, a former division chief at the U.S. State Department, told ABC News on Sunday, "We walked over to the window and we were suddenly surprised to see row after row after row of centrifuges. The North Koreans say it was 2,000. It was a lot." He added, "I'm not kidding you, I think our minds went blank for a second because it was so stunning."

Carlin said it demonstrated to him "that the policy that we've been following seems to be at a dead end."