STOCKHOLM — STOCKHOLM (AP) — Swedish authorities are investigating a cargo ship sailing in the Baltic Sea that they believe is a stateless vessel.

The Caffa was sailing in Swedish territorial waters Friday under a Guinean flag when officials felt suspicious of the craft, Swedish police said in a statement.

The Swedish Coast Guard boarded the vessel to search it and conduct interviews. Authorities were concerned about its seaworthiness.

Other details about the Caffa were not immediately available.

Sweden last year said it would step up insurance checks on foreign ships in a move aimed at tightening controls on Russia’s so-called “shadow fleet ” of aging ships. Russia uses its shadow fleet to transport oil and gas or to carry stolen Ukrainian grain. The average age of the vessels is around 18 years, meaning they’re near the end of their life span and are more vulnerable to accidents, especially if they’re not well-maintained.

The government in Stockholm last year tasked the Coast Guard and the Swedish Maritime Administration with collecting insurance information not just from ships that call at Swedish ports, but also those that pass through the country’s territorial waters and exclusive economic zone.

Authorities have not said whether they believe the Caffa is part of the shadow fleet.