Top Russian officials acknowledged for the first time that a ship hijacked in the Baltic Sea might have been carrying a suspicious cargo, deepening the mystery around its seizure.
Speculation has been raging that the Arctic Sea - seized by pirates near Sweden last month and missing for weeks before its recapture by the Russian navy in the Atlantic - may have held weapons or even nuclear materials.
The Maltese-flagged vessel with a crew of 15 Russian sailors was officially heading to Algeria with a cargo of timber. But Moscow's top investigator, Alexander Bastrykin, cast doubt on that theory.
"We do not rule out the possibility that the Arctic Sea transported something other than wood," Bastrykin told the official government newspaper Rossiyskaya Gazeta.
"This is why we asked the crew to remain in Moscow, as we must figure out if any one of them was involved in those events," added Bastrykin, who heads the investigative committee of Russian prosecutors.
The reported detention of 11 Arctic Sea sailors by Russian authorities and claims that they are prohibited from communicating with their families have fuelled speculation of a cover-up.
Strangely, just hours after Bastrykin's interview was published, his press service issued a statement denying that the ship had been on any "secret mission" or that it had been carrying illegal materials.
"The investigation currently does not have any information that the ship could have carried any sort of illegal cargo," it said.
Its statement also rejected allegations of a cover-up and said the sailors were not being kept in isolation.
Meanwhile, the head of the Russian military said that Moscow still needed to clarify what exactly was on board the 4,000-tonne ship.
"We do not know what it is carrying, we only know there is wood and whatever else it is carrying must be clarified by the investigation," Nikolai Makarov, chief of Russia's general staff, said during a visit to Mongolia.
In addition, the motive behind the hijacking - in an elaborate operation in one of Europe's busiest shipping lanes - "is simply not very clear," he added.
Two Russians, four Estonians and two Latvians are detained in Moscow on suspicion of hijacking the Arctic Sea, after being arrested when Russian warships recaptured the vessel off the Cape Verde islands.
Until this morning, Russian officials had repeatedly said that the vessel was carrying nothing sensitive.
Moscow's ambassador to Nato, Dmitry Rogozin, has said it was only carrying timber and dismissed speculation that it was carrying weapons as "tales".
But numerous theories have cropped up about the ship's cargo and its destination.
This week the Russian tabloid Moskovsky Komsomolets, citing Russian security sources, said the ship was smuggling arms and that the hijackers were stooges hired by the intelligence service of an EU member state to intercept it.
Russian officials have said that a preliminary search of the ship after it was recaptured found nothing suspicious, but they vowed a more thorough search when it reaches the Russian port of Novorossiisk.
Authorities in Finland, which the ship departed from on July 23, have said it was not carrying any radioactive cargo.
Still, the Arctic Sea mystery has revived fears - dramatised in Hollywood movies of the 1990s - that nuclear arms smuggled from the former Soviet Union could fall into the hands of terrorists or rogue states.
Russia has usually dismissed such concerns and argued that its nuclear arsenal is secure.