The ship, its 15 Russian crew members and its euro1.3 million ($1.8 million) cargo of timber had been due to make port Aug. 4 in Algeria but haven't arrived.
Speculation on what might have happened has ranged from theories that it might have been carrying secret cargo to the possibility that it fell victim to an almost unheard of case of sea banditry in European waters.
"If this is a criminal act, it appears to be following a new business model," Marine intelligence expert Graeme Gibbon-Brooks told Sky News on Wednesday.
Malta Maritime Authority said the MV Artic Sea "has not approached the Straits of Gibraltar, which indicates that the ship headed out in the Atlantic Ocean."
Russian President Dmitry Medvedev ordered that "all necessary measures" be taken to find the missing ship, and the Russian navy turned all of its vessels in the Atlantic -- including three landing ships, a frigate and two nuclear-powered submarines -- to search.
Finnish police said they also were assisting in the investigation.
The ship had reported being attacked July 24 off the Swedish island of Oland. It said a group of 10 masked men had boarded, tied up the crew and beat them before leaving 12 hours later in a high-speed inflatable boat, Swedish police said.
The Arctic Sea, having begun its journey in Finland, said the masked men identified themselves as police officers -- but Swedish police said they hadn't searched any ships in that area.
"We were very puzzled when we first heard about this," Swedish police investigator Ingemar Isaksson said at the time. "I have never heard of anything like this in Swedish waters."
On July 28, the Arctic Sea made routine contact with British maritime authorities as it passed through the busy Dover Strait, one of the world's busiest shipping lanes.
The ship offered what appeared to be a routine report -- identifying itself and its cargo, and saying where it had come from and where it was going, said Mark Clark of Britain's Maritime and Coastguard Agency.
He said the agency is "extremely curious" about what happened to the ship. "There is no coastguard I know who can remember anything like this happening," Clark said.
Where the ship was next spotted is uncertain. Russian media reports say the last contact was on July 30 when the ship was in the Bay of Biscay, and that it was later spotted by a Portuguese patrol plane, but there was no contact.
But Portuguese Navy spokesman Commander Joao Barbosa said "we can guarantee that the ship is not in Portuguese waters nor did it ever pass through Portuguese waters."
The cargo was shipped by Finnish wood supplier Rets Timber, which also said it had no information about the ship's whereabouts.
Experts guessed the ship may have may have been carrying a secret cargo or that it might be at the center of a commercial dispute. They were wary attributing the disappearance to bandits, noting that though piracy is rife in waters off lawless Somalia and in other areas of the world, European waters have been free of such attacks for centuries.
"There have been no attacks in European waters," said Pottengal Mukundan, director of the London-based International Maritime Bureau. "It's not the kind of area where pirates would find it easy to operate."
Maritime expert Mikhail Voitenko, editor of Russia's Maritime Bulletin, speculated the ship carried "a certain mysterious cargo," such as very valuable or hazardous materials, "and a certain third party, having seized the ship, was determined at any cost to make sure the cargo did not reach its recipient," according to a report posted on the Maritime Bulletin's Web site.
The head of Merchant Maritime Warfare Centre told the BBC that, if anything had happened to the ship, cargo would have been found.
"I strongly suspect that this is probably a commercial dispute with its owner and a third party and they've decided to take matters into their own hands," Nick Davis said Wednesday.
Pirate attacks off Somalia's lawless coast are a far more familiar occurrence. Pirates have launched more than 100 attacks this year in the Gulf of Aden and are now holding about a dozen vessels.
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