Mr. Ryu and his fellow spy, Ri Tae-gil, were first arrested in Ukraine back in 2011 for trying to steal missile secrets while posing as members of a North Korean trade mission. Ukraine’s domestic intelligence agency, known as S.B.U., says it announced their arrest at the time — I have found no record of this — but that nobody paid any attention until this year, when North Korea shocked the world by launching a missile capable of hitting the continental United States.

More shocking for Ukraine, however, was a July report in The Times that North Korea may have accelerated its previously problem-plagued missile program by stealing missile technology from a Soviet-era rocket factory in eastern Ukraine. Ukrainian officials dismissed the report as Russian disinformation and, in an effort to prove that North Korea could never have obtained secret technology in Ukraine, alerted journalists to the presence of the two North Korean spies in Zhytomyr.

CNN first reported on their imprisonment in early September, and Mr. Hoffman and I made our own trip to Zhytomyr to see them a few days later with help from Oleksandr Turchynov, the head of Ukraine’s security and defense council. That Ukraine has two North Korean spies in jail, Mr. Turchynov told me, showed that his country took a firm stand against missile and nuclear proliferation and would never have allowed secret technology to leak to Pyongyang.

The clearly terrified prisoners, however, had no interest in playing along with Ukraine’s efforts to clear its name. Neither expressed any complaints about their confinement, but, apparently haunted by what awaits them back in North Korea, both resembled hunted animals.

Unlike Mr. Ryu, the first of the jailed pair with whom I tried to speak, Mr. Ri — a second, older spy — did not run away when presented with a journalist. But he declined to say anything of substance about missiles, mumbling in Russian to answer questions, and then quickly demanding that guards take him back to his cell.