Swedish police confirmed on Tuesday night that they were investigating a bizarre series of events in which a ship reported an alleged pirate attack off Sweden’s Baltic islands and then disappeared somewhere off western Europe.

The 15 Russian crew of the Arctic Sea, a Maltese-flagged general cargo ship heading from Jakobstad in Finland to Bejaia in Algeria, reported being tied up and beaten by 10 men who boarded the ship at about 3am on July 24 off the Swedish island of Gotland, according to Ylva Voxby, a press officer for Sweden’s national criminal police.