Shipping experts have ruled out the possibility that the Maltese-flagged Arctic Sea, which has been missing for more than two weeks, has sunk.
Floating wood and oil would have been spotted by now, giving the sinking away, they said.
Russian naval vessels and satellites are scanning the Atlantic Ocean after it was revealed that the 3,988-tonne Russian-owned cargo ship with Russian crew on board did not go through the Straits of Gibraltar.
In fact, maritime authorities in Gibraltar have insisted that there was no record of the Arctic Sea "coming to or coming near Gibraltar", and the ship is believed to have headed out to sea.
Speculation is rife about what could have happened to the ship, which disappeared off the radar after passing through the English Channel carrying some €1 million worth of timber from Finland to Algeria.
The ship, with 15 Russian crew on board, had been attacked by a dozen pirates purporting to be anti-drugs police. They allegedly assaulted, tied, gagged and blindfolded the crew.
The editor of Russia's Sovfracht maritime bulletin, Mikhail Voitenko, has suggested that the ship was attacked because it was carrying a secret shipment of something else. He said he did not believe that pirates would be after a cargo of timber, and was likely carrying a secret cargo of arms or drugs.
Most likely, the Russian crew was caught in a mafia feud over illicit goods, he said.
"I think this must be linked to an attempt to covertly ship some kind of secret cargo, and someone really didn't want this cargo to get to its destination," Voitenko said.
Yesterday the Russian navy denied reports that one of its frigates was following a ship similar to the Arctic Sea in the Atlantic Ocean. Mr Voitenko had told a news channel that the navy's frigate, Landy, was pursuing a similar vessel south of Gibraltar.
The Kremlin has ordered Russian warships to join in the hunt for the 98-metre bulk carrier which was supposed to have docked in the Algerian port of Bejaia on August 4.
"The information is being reported to Russia's naval command and is constantly being analysed," a navy spokesman was quoted saying by the Interfax news agency.
Russian President Dmitry Medvedev has ordered the navy "to take all necessary measures to find and free" the Arctic Sea and its crew.
Yesterday a Finnish blogger said that an unidentified vessel the same length as the Arctic Sea had arrived in the Spanish port of San Sebastian. But port authorities denied this rumour, saying that they did not dock cargo ships.
The Arctic Sea is linked to an automatic tracking system but the last signal was received on July 30, showing it was off the coast of northwestern France.