The circumstances of the vessel's sudden sinking were suspicious, prompting the maritime captaincy to begin questioning the crew. Ursa Major's master, Capt. Igor Vladimirovich Anisimov, initially told investigators the cargo consisted of more than 100 empty containers, two giant crawler cranes on deck, and two large components for a Russian icebreaker project, referring to the tarped objects near the stern. All cargo was reportedly bound for Vladivostok.

The two so-called "icebreaker components" were shipped as deck cargo and were visible to spotting aircraft during the ship's earlier transit. Based on aerial surveillance, each object measured approximately 20 to 25 feet square, including crating, dunnage, and tarping.

Spanish authorities estimated their weight at roughly 65 tonnes each, indicating unusually high density. La Verdad reported that after the captain was pressed on the matter, he asked for time to think before telling investigators the items were merely "manhole covers."

Documents reviewed by La Verdad show Spanish investigators ultimately identified the cargo as casings for nuclear submarine reactors, specifically two Soviet-era VM-4SG reactors.

As for the destination, Spanish authorities speculated the reactor components may have been intended for North Korea's nuclear submarine program, which recently unveiled its first ballistic-missile submarine. Multiple analysts have suggested the new North Korean vessel likely benefited from Russian technical assistance for reactor design and could potentially incorporate a fully built Russian reactor. Russia is believed to owe North Korea a strategic debt following Pyongyang's large-scale transfers of artillery shells and munitions that helped Russian forces stabilize and regain ground in eastern Ukraine.

The cause of Ursa Major's sinking appears to have been kinetic. The shipowner told media there were three explosions and a 20-inch hole in the shell plating, while the captain confirmed the hole's ragged edges were bent inward. This damage profile is consistent with an external explosion impacting the hull.