But Siegfried Hecker -- the former director of the American nuclear research center in Los Alamos, in the southwestern state of New Mexico -- made it clear that he was not part of an official inspection team when he visited North Korea's nuclear facility in Yongbyon on 8 January.
Hecker said that after his tour of the site, he told his North Korean hosts that he doubted they could make what the North Koreans call a "nuclear deterrent" because they do not have all of the three elements in place to do so: the ability to make plutonium metal, the ability to include it in a nuclear bomb, and the ability to integrate it into a device that can deliver it to its target.
The North Koreans, Hecker said, made what he called "a convincing case" that they were capable of producing plutonium, but not a delivery system. Most crucial, he said, was that the North Koreans did not demonstrate that they have the equipment or the expertise capable of building the nuclear bomb itself.
"You have shown me no facilities. You have not shown me anyone that I could talk to, who would give me any indication whatsoever that you can build a nuclear device," Hecker said.
On the other hand, Hecker said he is convinced that North Korea not only is capable of reprocessing and enriching uranium to make what he called plutonium -- which is needed to make nuclear bombs -- but he also believes that they already have done so.
Hecker said 8,000 spent fuel rods from the Yongbyon were missing. He said he checked the area where they were supposed to be and found nothing. Then, he told the committee: "I asked them, of course, 'What did you do with them?' They told us, they said, 'We reprocessed them. And we not only reprocessed them, but we reprocessed them to make plutonium metal.'"
Hecker said that his conversations with various nuclear scientists at the facility convinced him that they had both the equipment and the expertise to create plutonium. He said he could not confirm this, but said the North Koreans were persuasive.
"The bottom line is, all of their answers were straightforward, and they were all technically sound," Hecker said.
The administration of U.S. President George W. Bush accuses Pyongyang of running a secret nuclear program in violation of a 1994 arrangement under which North Korea's Communist Party general secretary, Kim Jong Il, was supposed to freeze that effort. Since then, the United States and its allies have suspended shipments of free oil to North Korea that were called for in the deal.
In questioning Hecker, Senator Joseph Biden, the vice chairman of the Foreign Relations Committee, said his concerns about North Korea go far beyond any immediate threat -- to the eventual use of Korean weaponry by terrorists.
"If [the North Koreans] added two, three, four, five, six more nuclear devices, that concerns me, but it concerns me less than if the material they have they could sell and/or export in a form that someone other than a nation state would find useable to construct a 'homemade' nuclear device," Biden said.
Biden said that even if such a nuclear bomb could not be made small enough to be delivered by a missile, it could be easily stowed in a ship's hold and detonated in a harbor of a coastal city such as New York.
Hecker and other members of his delegation have been briefing officials with the Bush administration since their return from North Korea. Another member of the committee, Senator Sam Brownback, welcomed this help and applauded the administration's concern for fighting nuclear proliferation.
But Brownback said the United States should not limit its involvement in North Korean affairs to its nuclear weapons programs at a time when Pyongyang is almost universally accused of broad human rights abuses. "I think you have to tie in the human rights portfolio with the issue of weapons development, because we've got suffering on the largest scale of anywhere in the world taking place today in North Korea, and we just can't continue to turn a blind eye to that," Brownback said.
In his closing remarks, Hecker addressed Brownback's concerns, but said nonproliferation efforts can be valuable in advancing human rights. The nuclear weapons scientist told the committee that what he called the "horrific consequences" of nuclear weapons must never be forgotten, and added, "Today we only have the distant but stark reminders of Hiroshima and Nagasaki. But these weapons are of a totally different type. When you release the energy of the nucleus, you're talking about a factor of millions compared to anything that can be done conventionally."