Mr. Elleman’s detailed analysis is public confirmation of what intelligence officials have been saying privately for some time: The new missiles are based on a technology so complex that it would have been impossible for the North Koreans to have switched gears so quickly themselves. They apparently fired up the new engine for the first time in September — meaning that it took only 10 months to go from that basic milestone to firing an ICBM, a short time unless they were able to buy designs, hardware and expertise on the black market.
The White House had no comment when asked about the intelligence assessments.
Last month, Yuzhmash denied reports that the factory complex was struggling for survival and selling its technologies abroad, in particular to China. Its website says the company does not, has not and will not participate in “the transfer of potentially dangerous technologies outside Ukraine.”
American investigators do not believe that denial, though they say there is no evidence that the government of President Petro O. Poroshenko, who recently visited the White House, had any knowledge or control over what was happening inside the complex.
On Monday, after this story was published, Oleksandr Turchynov, a top national security official in the government of Mr. Poroshenko, denied any Ukrainian involvement.
“This information is not based on any grounds, provocative by its content, and most likely provoked by Russian secret services to cover their own crimes,” Mr. Turchynov said. He said the Ukrainian government views North Korea as “totalitarian, dangerous and unpredictable, and supports all sanctions against this country.”
How the Russian-designed engines, called the RD-250, got to North Korea is still a mystery.
Mr. Elleman was unable to rule out the possibility that a large Russian missile enterprise, Energomash, which has strong ties to the Ukrainian complex, had a role in the transfer of the RD-250 engine technology to North Korea. He said leftover RD-250 engines might also be stored in Russian warehouses.
But the fact that the powerful engines did get to North Korea, despite a raft of United Nations sanctions, suggests a broad intelligence failure involving the many nations that monitor Pyongyang.
Since President Barack Obama ordered a step-up in sabotage against the North’s missile systems in 2014, American officials have closely monitored their success. They appeared to have won a major victory last fall, when Mr. Kim ordered an end to flight tests of the Musudan, an intermediate-range missile that was a focus of the American sabotage effort.
But no sooner had Mr. Kim ordered a stand-down of that system than the North rolled out engines of a different design. And those tests were more successful.
American officials will not say when they caught on to the North’s change of direction. But there is considerable evidence they came to it late.
Leon Panetta, the former C.I.A. director, said on CBS’ “Face the Nation” on Sunday that the North Korean drive to get workable ICBMs that could be integrated with nuclear weapons moved more quickly than the intelligence community had expected.
“The rapid nature of how they’ve been able to come to that capability is something, frankly, that has surprised both the United States and the world,” he said.
It is unclear who is responsible for selling the rockets and the design knowledge, and intelligence officials have differing theories about the details. But Mr. Elleman makes a strong circumstantial case that would implicate the deteriorating factory complex and its underemployed engineers.
“I feel for those guys,” said Mr. Elleman, who visited the factory repeatedly a decade ago while working on federal projects to curb weapon threats. “They don’t want to do bad things.”