Stockholm - Swedish police Friday treating very seriously reports of a mysterious presumed pirate attack on a Maltese- registered freighter in the Baltic Sea.

Swedish deputy crime police chief Tommy Hydfors confirmed to the German Press Agency dpa the report that eight heavily-armed masked men on Friday last week boarded Finnish-owned freighter Arctic Sea from a rubber dinghy.

According to the ship's 15-member Russian crew, the armed men claimed to be drug enforcement agents and thoroughly searched the ship, reacting violently to anyone who got in their way, including using a rifle butt to knock out teeth from one crew member.

The ship was held for 12 hours before the hijackers men left again without taking anything.

Investigators speculated that the "pirates" may have actually been a drugs gang that was acting on a tip to search for contraband.

Investigator Ingemar Isaksson was quoted by the Swedish daily Expressen as saying there were a number of open questions, including why the crew waited several days before reporting the incident to the ship's owners Solchart Management.

Investigators however said they was no indication of a possible a rise in piracy in Swedish waters, which had not seen a single incident of piracy in modern times.

The Arctic Sea, which was carrying a shipment of wood from Finland to Algeria, continued its journey following the reported incident. (dpa)