Russia has stolen hypersonic cruise missile technology from the United States, U.S. President Donald Trump’s top national security aide John Bolton said while discussing last week’s deadly explosion at a military test site in Russia’s north.

The mysterious explosion that killed five Russian nuclear engineers and caused a radiation spike last Thursday has spawned conflicting theories over its cause. Amid high secrecy and the absence of thorough explanations from Russian authorities, outside observers have concluded that the explosion occurred during testing of a nuclear-powered cruise missile.