The world is wondering why North Korea hasn’t test-fired another missile in more than eight weeks. But satellite photos suggest that Pyongyang is busy making its rocket engines better.

Commercial satellite imagery from mid-September to mid-October analyzed by 38 North hints that North Korea might have tested a new solid-fuel rocket engine at its Magunpo Rocket Engine Test Facility sometime between October 15-21.

Analyst Joseph S. Bermudez Jr. says the photos show activity at the site “that may be indicative of engine testing” and that matches the timing of a suspected test reported by Ankit Panda in an earlier article for the Diplomat. Panda said his information came from US government sources.

“However, the nature of this activity (excavation, presence of debris, etc.) cannot conclusively confirm or negate the event,” Bermudez wrote in the article for the specialist website on North Korea hosted by Johns Hopkins University.

But Bermudez notes that the Magunpo facility is a key component in North Korea’s programs to develop and test all classes of solid-fuel rocket engines and has been operational since late 2014.

North Korea hasn’t conducted another missile test since its Hwasong-12 intermediate range ballistic missile launch on September 15. It tested two long-range Hwasong-14 ICBMs in July.

Some analysts say that Pyongyang will make another high-profile missile test to prove that it has the ability to hit New York or Washington with a nuclear warhead.