A ransom demand has been received for the return of a Russian-manned freighter that went missing last month in the Atlantic, Finnish investigators said Saturday.

It was not immediately clear if the demand was legitimate, and there are no answers on the whereabouts of the Arctic Sea, its 15 crew members or its cargo.

The crew had said they were attacked in Swedish waters four days before the ship disappeared on July 28, but there has been no confirmation that the ship was actually seized.

"A ransom demand has been made," Markku Ranta-Aho, of Finland's National Bureau of Investigation, told national YLE radio.

When asked how much the ransom was for he answered, "Let's say it's a largish amount of money."

The demand was addressed to the Finland-based company that owns the Arctic Sea, but to protect the crew, Markku Ranta-Aho refused to provide details on either the ransom note or the whereabouts of the ship.

Reported off West Africa

The French Navy said Saturday the ship was likely near Cape Verde. Widespread reports on Friday also had placed the ship near the island nation off West Africa.

Cape Verde authorities said they had no new information Saturday.

Russian maritime Web site Sovfrakht said the ship's tracking system had sent signals on Saturday from the Bay of Biscay, some 3,200 kilometres north of Cape Verde. It cautioned, however, that the Arctic Sea's Automatic Identification System equipment may not be on the ship itself anymore.

The signals disappeared after about an hour, it said.

The Arctic Sea had set out from Finland on July 23 and was due in an Algerian port on Aug. 4. It vanished on July 28 after passing through the English Channel.

Efforts to pinpoint its location have been difficult in the vast Atlantic and there has been no communication from the ship's 15-member Russian crew.

Speculation on what might have happened has ranged from suspicions that the ship was carrying secret cargo, possibly narcotics, to theories about a commercial dispute.

Security experts have been wary of attributing its disappearance to bandits or hijackers, noting that piracy is almost unheard of in European waters.