WASHINGTON (NYTimes) - In the remote North Korean city of Hamhung, separated from the capital by vast, jagged mountains, an inconspicuous chemical plant may be secretly fuelling the growing missile array that threatens the United States.
Researchers think that the plant is producing a specialised rocket fuel known as UDMH, which is used in the long-range missile launches that have escalated tensions between North Korea and the United States.
This would settle an esoteric but crucial debate among North Korea watchers, and not to Washington's favour.
Some have argued that North Korea cannot produce the fuel, implying that the country imported it from Russia or China. Those countries could then be pressured to cut off exports, grounding North Korea's missiles without firing a shot.
But the new finding, produced by the James Martin Center for Nonproliferation Studies at Middlebury University, suggests that North Korea has mastered UDMH production, closing off one of the last avenues for outside curbs on the country's increasingly sophisticated weapons programmes.
Though North Korea may have previously relied on foreign assistance in obtaining or making the fuel, as some analysts believe, it no longer appears to need the help.
Vipin Narang, a Massachusetts Institute of Technology professor who studies nuclear issues, called the discovery "very important".
"If they are not dependent on foreign suppliers," he said, then "even the most targeted sanctions on, and monitoring of" countries that might assist North Korea, "will be mostly futile."
Short of war or the country's collapse, he added, "There's nothing to stop this programme from becoming a monster."
The finding is based on satellite imagery, a technical analysis of UDMH production methods, information from a North Korean official who defected, and a set of obscure North Korean technical documents.
Mr Jeffrey Lewis, who directs the Middlebury center's East Asia programme, had been hunting for weeks for hints of UDMH production.
"There are no real, obvious signatures for it," he said, because it can be made with common chemicals like chlorine and ammonia using a variation of a process developed in 1906. India, while quietly developing its missile programme in the 1970s, had produced UDMH in an old sugar factory.
The breakthrough came when his team found and translated a set of highly technical articles in an official North Korean science journal, Chemistry and Chemical Engineering, that referred to the fuel.
The articles, which ran between 2013 and 2016, discussed mundane matters like managing highly toxic wastewater, a notorious problem in UDMH production. But they betrayed suspiciously sophisticated knowledge. One explored methods for improving purity, crucial in advanced missiles.
"They don't read like this is a speculative or nascent endeavour," Lewis said.
"They read like this is a problem they've been working on for a while," describing problems a country would encounter only after producing large quantities of the fuel.
But the documents betrayed something else: conspicuous secrecy. Unlike others in the journal, these three articles listed no affiliation or biography for its authors, suggesting their work was more sensitive than it looked.
Lewis' team cross-referenced the authors' names with every scrap of North Korean chemical research they could find, until they noticed something odd. One of the authors, Cha Seok Bong, had published three papers on more anodyne matters out of a place called the February 8 Vinalon Plant, based in remote Hamhung.
It was an odd location for a highly trained rocket fuel specialist to work. The plant normally produces vinalon, a cheap synthetic material sometimes called "juche" fiber - a reference to North Korea's tenet of juche, or self-reliance - that is often used in North Korean textiles and uniforms.
But it had long been, Lewis said, "our No. 1 candidate for UDMH production."
His team had initially flagged the plant after scrutinising, in painstaking detail, satellite images of Hamhung for clues.
The remote city is not an obvious home for sensitive military sites.
Sitting on the country's eastern coast, it is exposed to airstrikes, like the US bombing missions that devastated it in the Korean War.
But Ko Chong Song, a North Korea official who defected in the early 1990s, indicated in a 2001 book that it was the center of secret military chemical work. The CIA had suspected as much since at least 1969, when it published a secret assessment of chemical production in Hamhung.
Now, Lewis' team, looking again at the plant, noticed two unusually large wastewater pools, which aligned with standard UDMH production methods - and with the paper describing wastewater challenges. And Kim Jong Un, the North Korean leader, they found, had made a series of trips to the plant, underscoring its importance.
"That's about as clear-cut as it gets," Lewis said.
While the finding reveals important information about the extent of North Korea's progress, it may come too late for the United States to act.
The country has most likely already stockpiled enough fuel to fight an extended war, Lewis said. And the fuel is designed to remain potent for years. Soviet UDMH lasted so long that, after the country collapsed, the United States had to help de-fuel its intercontinental ballistic missiles.
Asked how North Korea could have so extensively developed this fuel without apparent outside notice, Lewis said outside analysts too often saw the country as primitive and backward.
"If you watch them in satellite photos and read their technical publications, it looks like a totally different country," he said.
He added, sighing, "We're in full-scale denial about North Korea's capabilities."
This is a column and newsletter by Max Fisher and Amanda Taub exploring the ideas and context behind major world events.