A small submarine that may be Russian has been discovered on the sea floor near the Swedish coast, according to a group that explores shipwrecks and artifacts in the area.

Ocean X Team said in a press release on Monday that the submarine was found last week. The group is not sure how long the vessel has been on the sea floor, but it does have letters on its side painted in the Cyrillic alphabet, indicating a likely Russian origin.

According to Ocean X Team:

The submarine is completely intact, has no visible damage to the hull and the hatches are closed. Therefore we do fear that the crew has not been able to save themselves when the sub went down, says Stefan Hogeborn, divers [sic] in Ocean X Team. The submarine was found during an expedition where a camera equipped, unmanned mini-submarine of the model ROV, Remote Operated Vehicle, was sent down. Swedish Armed Forces was informed earlier today.

A Swedish news outlet posted pictures of the discovery.

The Finnish Navy dropped depth charges in May near Helsinki in response to the possible presence of a Russian submarine. Eastern European countries have become increasingly concerned about Russia’s aggressive actions in the continent’s coastal waters and its bomber over-flights in other countries’ airspace.