MOSCOW (Reuters) — Russia said nothing suspicious was found on board the Arctic Sea ship, whose seizure by suspected pirates off the coast of Europe this month sparked a media storm.

Media reports had speculated that the ship had a secret cargo of arms or even nuclear materials, saying a ship carrying timber in some of the world's best policed seas would be an unlikely target for pirates.

"The initial inspection of the ship did not uncover any suspicious cargo," Russia's Foreign Ministry said in a statement.

The Foreign Ministry said the captain initially said the ship was North Korean, but this information was denied by Pyongyang.

Russian prosecutors say the ship, which had a mainly Russian crew was operating with a Maltese flag when it was seized off the coast of Sweden.

Russian authorities have questioned crew members after the seamen and eight alleged pirates were returned to Moscow, adding new details to the mystery of the ship's monthlong odyssey.

Three heavy-lift air force jets, reportedly carrying 11 crew members, the alleged hijackers and other investigators, arrived at a Moscow region military base after flying from Cape Verde, the West African island nation where a Russian frigate stopped the ship last week.

State television channel Rossiya said the suspects claimed to be were ecologists who had been arrested by mistake.

The saga of the Maltese-flagged freighter named which left Finland on July 21 carrying a load of timber to Algeria, has gripped much of Europe.

The ship was found nearly two weeks after it was to have docked in Algeria, thousands of miles off course and long out of radio contact.

Speculation on what was behind the freighter's diversion was heightened by the involvement of the Russian navy, the slow trickle of information and claims that news media were fed bogus information about the ship. It was unclear why three planes were needed to fly such a small group of people to Moscow, nor why Il-76s — among Russia's largest planes — were used for the operation.

Federal investigators said in a statement that crew members told them that, while the Arctic Sea was in Swedish waters, the ship was boarded by eight men who wore uniforms that read "POLICE" on the back and who threatened the crew.

The statement did not give more details about the seizure or say if the men left the ship 12 hours later as earlier had been reported. The Interfax news agency said the 11 crew members and hijackers were taken to Moscow's notorious Lefortovo prison, run by the main KGB successor agency.

It cited an unidentified law enforcement official as saying the crew members will be freed if the investigators confirm they were not involved in the hijacking.

Russian TV channels showed footage of what it said were air force planes arriving at the Chkalovsky base near Moscow, and men believed to be the hijackers being escorted roughly by special forces troops.

Rossiya said the alleged pirates identified themselves as ecologists when they were arrested.

But in footage from the hold of one of the planes, a suspect identified as Andrei Lunev was asked what ecological organisation he was connected with.

"I don't know," the man said. He also denied that he or the other suspects were armed and said they went aboard the Arctic Sea to ask for gasoline for their inflatable vessel.

Men identified as Arctic Sea crew members told Vesti that the ship was seized in the Baltic Sea by gunmen.

One unidentified man told Vesti that a crew member sent a text message saying the ship had been hijacked, but the hijackers then forced the captain at gunpoint to report that everything was normal on board.

Vesti also showed men in handcuffs, whom it identified as the suspected hijackers, being led by Russian marines to buses in Cape Verde.

Russia has said four were citizens of Estonia, and the others were from Russia and Latvia.

There were conflicting statements about the fate of the Arctic Sea and its cargo of timber.

The Foreign Ministry said that the ship's captain and three remaining crew members stayed behind to help in return the ship to its owner.

The Kremlin said the ship was en route to the Russian Black Sea port of Novorossiisk, but federal investigators said it was drifting off Cape Verde.

More than a week after the Arctic Sea's departure from Finland on July 21, Swedish police said they had received a report that masked men had raided the ship in the Baltic Sea and beaten the crew before speeding off 12 hours later in an inflatable craft.

The freighter gave no indication of any difficulties or change in its route during radio contact while passing through the English Channel on July 28.