(CNN) -- A ransom has been demanded for a missing Russian cargo ship which vanished two weeks ago after being involved in a suspected hijacking off the coast of Sweden last month, Finnish police told CNN Saturday.

The last known contact with the Arctic Sea was July 31. Mystery surrounds its movements and the fate of its crew.

Authorities said the ransom demand might be from a second group of hijackers who targeted the ship after it was first allegedly hijacked for 12 hours off the coast of Sweden.

"There has been a demand for ransom and the subject is the shipping company, Solchart Management," Jan Olof Nyholm with the Finnish police told CNN.

An international criminal investigation is under way into the alleged hijacking of the vessel, the Arctic Sea. The last known communication with the vessel was July 31.

The probe, involving Interpol, is being handled by Swedish, Maltese and Finnish authorities in cooperation with authorities from another 20 countries, the Malta Maritime Authority said in a statement Saturday.

Finnish police would not say how much ransom had been demanded, or what else they know about the vessel.

"At this point I can't comment on whether we know the whereabouts of the ship. Our top priority is the threat to life and health, so I can't release any more details," Nyholm said. "The investigation has more details, but I can't say any more."

The Arctic Sea, which sails under a Maltese flag, was carrying a 6,500-ton cargo of timber from Finland to Algeria when it reported trouble on July 24.

Its 15-member crew told authorities that eight to 12 people armed with guns and pistols boarded the vessel about 3 a.m. that day, masked and wearing uniforms with the word "police" written on them, the Malta Maritime Authority said.

"During (the attackers') stay onboard, the members of the crew were allegedly assaulted, tied, gagged, and blindfolded and some of them were seriously injured," the maritime authority said in a written statement.

"All crew members were 'hard' questioned for a considerable amount of time. The questioning was related to drug trafficking. Later all crew members were released from their bindings but were locked within cabins until the alleged police rummaged the vessel thoroughly."

The attackers left after 12 hours on the same black rubber boat on which they had arrived, which bore the word "police," the maritime authority said.

The vessel's radar and satellite systems were off-line for two hours during the reported hijacking, during which it was witnessed performing "extreme maneuvers," said Maria Lonegard, a spokeswoman for the Swedish police.

The Finnish shipping company in charge of the ship reported the case to the Finnish police, who referred it to the Swedish police, the Maltese Maritime Authority said. In the meantime, the ship sailed through the English Channel.

Three days later, on July 31, Swedish police reached the ship by phone and spoke with someone they believed to be the captain, Lonegard said.

The crew provided photos of their injuries and written statements about the alleged hijacking, Swedish police said.

Despite that evidence, however, authorities have been unable to confirm the alleged hijacking. Swedish police say they have spoken to a number of witnesses who saw the ship making strange movements, but no one saw the black rubber boat approaching or leaving the Arctic Sea.

The ship has not been heard from since July 31. It did not arrive in North Africa as scheduled August 4.

Authorities have had no explanation for the ship's disappearance until Saturday, when Swedish police said they believe the Arctic Sea has been hijacked a second time. Track reports of the ship's journey »

"It appears that we are now dealing with two separate incidents -- the alleged hijacking off the Swedish coast and now the alleged hijacking with a demand for ransom," Lonegard told CNN on Saturday.

"The incident in the Swedish waters appeared over when we spoke to the crew and shipping company on the 31st of July. So it appears the ship has been hijacked twice."

Police don't know the location of those demanding the ransom, she said.

An international criminal investigation is under way into the alleged hijacking, led by the Swedish, Maltese and Finnish authorities in cooperation with authorities from another 20 countries, the Malta Maritime Authority said in a statement Saturday.

Interpol and Europol are also involved.

The ship was reported Friday to be in international waters north of Cape Verde, an island nation a few hundred miles from the coast of western Africa.

The news came from Portugal's state news agency, which quoted Cape Verde's defense director, Pedro Reis.

The U.S. military also had a report this week that the ship was seen a few hundred miles from Cape Verde, two military sources told CNN, but the United States had no independent verification of those reports. The U.S. military is not involved in the search.

Russia's ambassador to Cape Verde, however, denied that the Arctic Sea had been spotted near the island.

The Russian military has been searching for days for the Arctic Sea, with naval vessels authorized to use force, Russia said this week.

Russian President Dmitry Medvedev instructed Defense Minister Anatoly Serdyukov to "take all necessary measures to locate, monitor and, if necessary, to free the missing vessel," a statement issued by the president's office said.

Experts say maritime crime is rare in heavily policed European waters and more common around areas, such as Somalia, where governments have little or no control over their ports.

"Attacks on ships are extremely rare; basically they don't happen," said Jeremy Harrison of the British Chamber of Shipping.

A spokesman for the Swedish Coast Guard said the last known hijacking of a vessel in Swedish waters occurred in the 16th century.

"The only way a ship can disappear is if someone has actually turned off the ship's beacon," said Natasha Brown with the United Nations' International Maritime Organization. "But if this is done, you could only find the ship if you actively searched for it with a plane or helicopter."

CNN's Per Nyberg in London, England, contributed to this report.

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