Spanish authorities said the ship’s captain later admitted the cargo included “components for two nuclear reactors similar to those used in submarines,” though he was unsure whether they contained nuclear fuel.

The reactors were likely headed to the North Korean port of Rason after Pyongyang sent troops to support Russia in Ukraine. The vessel slowed suddenly on December 22 before suffering three explosions near the engine room a day later, killing two crew members.

Spanish investigators found a 50cm inward-facing hole in the hull and are examining whether a rare supercavitating torpedo or explosive device caused the damage. After the surviving crew evacuated, the Russian warship Ivan Gren ordered nearby vessels away before four more explosions were detected shortly before the ship sank.

A week later, the suspected Russian spy ship Yantar visited the wreck site, and four additional explosions were reportedly recorded.

The wreck has since attracted unusual military attention, including two flights by rare U.S. WC-135R “nuke sniffer” aircraft over the site. The ship’s owner later described the sinking as a “targeted terrorist attack.”