U.S. intelligence officials believe that at one time North Korea was obtaining a rare but potent fuel to power its missiles from China and Russia, but now may be capable of producing the fuel domestically.

At present, the U.S. government is attempting to learn whether Beijing and Moscow are continuing to supply the rare fuel, known as UDMH — unsymmetrical dimethyl hydrazine — and if so, whether that supply route can be sanctioned or sabotaged.

Though intelligence estimates dating back to the George W. Bush and Barack H. Obama administrations correctly assessed that North Korea’s missile development was maturing, there is no publicly available evidence to suggest that the U.S. ever moved to interdict Pyongyang’s supply of UDMH.

Some analysts believe it may be moot at this point, owing to the potential that North Korea may well have developed the capability to produce the highly toxic fuel itself. But others doubt that, given the complexity of manufacturing the fuel and its volatility; even in highly developed nations the manufacturing process has often produced explosions of missiles and at factories where the fuel is made.

The U.S. stopped manufacturing the fuel in the 1960s, opting to use solid fuels to power its nuclear-tipped ICBMs. Russia only recently began manufacturing it again, and a few other European countries make it. But China is the largest producer of UDMH.

The missile launch that took place on Friday, which was lofted over northern Japan, was fueled by UDMH, spy satellites showed.

And while China has regularly denied assisting North Korea’s missile programs, a 2008 secret report by the Missile Technology Control Regime and released by WikiLeaks states that Beijing has an “uneven track record in enforcing its missile-related export controls.”

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