In the remote North Korean city of Hamhung, separated from the capital by vast, jagged mountains, an inconspicuous chemical plant may be secretly fueling the growing missile array that threatens the United States.

Researchers think that the plant is producing a specialized 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 favor.

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.