(CNN) -- An international search was under way Wednesday for a cargo ship that vanished after its crew reported they were hijacked -- at least briefly -- nearly two weeks ago.
The last known contact with the Arctic Sea was July 31. Mystery surrounds its movements and the fate of its crew.
The Russian-crewed Arctic Sea, carrying a 6,500-ton cargo of timber from Finland to Algeria, was last heard from on July 31, when crew members spoke to Swedish police.
Its crew has told authorities that eight to 12 masked people posing as drug enforcement officers had boarded the vessel and bound and beat its crew in the waters off Sweden on July 24, according to a statement from the Maltese Maritime Authority.
The Arctic Sea sails under a Maltese flag.
"All of our staff in our company are working very hard to get back in contact with the ship," Victor Matveev, director of Solchart Management AB, the company that owns Arctic Sea, told CNN on Wednesday night. "It has been a really hard time for us."
On Wednesday, Russia said naval vessels authorized to use force were searching for the Arctic Sea with the aid of "space-based" detection systems. The Maltese Maritime Authority said Wednesday that the ship appears to have headed into the Atlantic Ocean, but mystery surrounded the ship's movements and the fate of its crew.
Russia's Defense Ministry said on its Web site that the Black Sea Fleet patrol ship Ladny was heading the search operation and had passed through the Strait of Gibraltar into the Atlantic.
Russian President Dmitry Medvedev has 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.
The ship's crew said its troubles began about 3 a.m. July 24, when the masked group boarded and stayed on board for about 12 hours, according to Swedish police and the Maltese Maritime Authority. Watch an account of the crew's alleged ordeal »
According to witness accounts, the intruders restrained the 15-man crew and questioned them about drug trafficking before locking them in their quarters.
"During their 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.
During the reported hijacking, the vessel's radar and satellite systems were off-line for two hours, during which time the ship was witnessed performing "extreme maneuvers," said Maria Lonegard, a spokeswoman for the Swedish police.
Also, Lonegard said, there were no witnesses who saw the black rubber boat the pirates were reported to have used. "We have no suspects and no witnesses," she said.
The "ship managers" reported the incident to police in Helsinki, Finland, on July 28, the Maltese Maritime Authority said. That same day, the ship made radio contact with the British Maritime and Coastguard Agency as it passed through the English Channel, but the crew reported no trouble, said the agency's Mark Clark.
Three days later, on July 31, Swedish police reached the ship by phone and spoke with someone they believe to be the captain, Lonegard said. It was the last known communication with the vessel, which was believed to be off the coast of France then.
Matveev said he spoke to the ship July 30, and "all looked normal, like normal day-to-day business." But he said he was unable to raise the ship August 1.
The ship failed to make its scheduled arrival in North Africa on August 4. Its last known position, Matveev told CNN, was off the coast of Portugal.
CNN tried to call the ship directly, but the ship's satellite phones appear to be turned off.
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.
The International Maritime Bureau in London, which tracks piracy worldwide, said it did not believe the Arctic Sea had fallen into the hands of pirates.
"We are not going to classify this as a piracy event, mainly because of the location and circumstance," spokesman Cyrus Mody said, adding that the bureau is unaware of any piracy in recent memory in the waters off Sweden.
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."
"The whole thing is more than strange, this is a first time in the history of our business," Matveev said.
CNN's Per Nyberg, Barry Neild, Alysen Miller, Laura Perez-Maestro, Chris Mansson, Maxim Tkachenko and Mark Bixler contributed to this report.