Moscow - Sweden, Malta and Finland have been called upon by Russia to help answer questions in the investigation into the hijacking and disappearance this summer of the "Arctic Sea" freighter.

Nikolai Makarov, chief of the General Staff of Russia's armed forces, told reporters Wednesday that many questions remain about the whereabouts, activities and cargo of the freighter.

Questions include: why did pirates initially approach the ship on July 24, and what was the freighter's cargo, if not wood as listed on its manifest.

"We don't know what it transported," said Makarov. "We'll know more after the inspections."

Vladimir Markin, head of Russia's investigative agency, said multiple countries will help in the investigation. Primary among those is Sweden, since the "Arctic Sea" initially disappeared in that country's waters.

The freighter was initially boarded by men claiming to be drug enforcement agents on July 24, before reportedly being freed again, only to disappear days later for almost three weeks before being sighted off the coast of Cape Verde. The boat's 15-man crew was eventually freed by Russian forces, who also took the eight hijackers prisoner.

Those men could now face 20 years in prison.

Rumours of alleged weapons smuggling have swirled around the ship ever since its initial disappearance. Citing European government officials, the Financial Times Deutschland has reported that the freighter is extremely low in the water, lower than would be expected if its cargo was purely wood.

The freighter is being towed to the Russian Black Sea harbour of Novorossiyski for further inspection. The 15-man crew will continue to be held until it can be ascertained that they had nothing to do with the hijacking.

The "Arctic Sea" might be held as evidence until a trial starts.