Sailors from a cargo ship allegedly hijacked by pirates in the Baltic Sea have gone home after their detention by Russian authorities fuelled theories of a cover-up, according to reports.

"According to my information, all 11 have returned home," Mikhail Voitenko, a shipping expert who has followed the case, said in a posting on his website, the Sovracht Maritime Bulletin.

A video posted on the tabloid news website Life.ru showed the sailors being joyfully greeted by their relatives after their train arrived in Russia's northern city of Arkhangelsk on Saturday evening.

Life.ru reported that nine sailors had returned instead of 11. The reason for the discrepancy was not clear.

The Russian sailors had been aboard the Maltese-flagged freighter the Arctic Sea on July 23 near the coast of Sweden when, investigators say, the ship was seized by a group of masked men posing as police.

The Arctic Sea was officially carrying a load of timber, but the boldness of the hijacking in a busy European shipping lane has stoked speculation that the ship held an illicit cargo, such as weapons or even nuclear materials.

Russian authorities said nothing suspicious was found aboard the ship when they recaptured it from the pirates earlier this month, but have vowed a more thorough inspection after it arrives in Russia's port of Novorossiisk.

After the ship's recovery, 11 of its 15 crewmen were flown to Moscow, while four others remained aboard to help guide the vessel back to shore.

Instead of a triumphant homecoming, the 11 sailors in Moscow were kept out of the media spotlight and family members complained that they were not being allowed to speak to them, fuelling theories of a cover-up.

Russian investigators said they had asked the sailors to remain in Moscow to determine whether any of them were involved in the crime, and denied holding them incommunicado or seeking to cover anything up.

The sailors appeared reluctant to speak to journalists upon their arrival in Arkhangelsk, Life.ru reported.

"As though by command, all members of the crew hid from cameras and declined to answer journalists' questions," the website wrote.

Eight suspected hijackers -- including Estonians, Latvians and Russians -- were detained by Russian authorities and are now being held in Moscow facing piracy and kidnapping charges. Their lawyers say the men are innocent.