MOSCOW, August 26 (RIA Novosti) - A Russian court has formally impounded the Finnish-owned Arctic Sea merchant vessel, which was allegedly seized by hijackers and freed by the Russian Navy, prosecutors said on Wednesday.

Vladimir Markin, spokesman for the top investigation committee of the Prosecutor General's Office, said the ship would have to be held to enable investigators to inspect it.

"Investigators plan to determine what cargo the vessel was carrying as part of a probe [into its disappearance]," Markin said.

The Arctic Sea, which had 15 Russian crewmembers, is currently being towed to the Russian port of Novorossiisk on the Black Sea. Four of the crew remain on board, while the other 11 have been flown to Moscow for questioning. Eight alleged hijackers, found on board when the freighter was released, have been arrested and are facing piracy charges.

He added that the vessel needed to be impounded in order to conduct an official and detailed inspection of the ship.

Press speculation on the mystery surrounding the Arctic Sea's disappearance has been rife with some media saying the Russian authorities are trying to cover up a smuggling or trafficking operation. Moscow-based columnist Yulia Latynina speculated that the vessel was secretly shipping arms via Algeria to a rogue state such as Iran or Syria.

But the Russian Foreign Ministry dismissed earlier such reports saying that a preliminary search of the vessel has been already conducted and no suspicious cargo has been found.

"The preliminary search of the ship did not reveal the presence of suspicious cargo on board. A more thorough search will be conducted at one of the ports on the way to Russia," the statement said.