SYDNEY, Australia - Up to three sharks may have been involved in the fatal attack on a 21-year-old woman off an island in eastern Australia, police said Sunday.

The woman, whose identity was not immediately released, died late Saturday after being attacked near North Stradbroke Island, east of the Queensland state capital Brisbane.

Queensland state police inspector Peter Harding said officers were searching for the sharks and had closed several nearby beaches.

The woman was swimming with a friends from a church group in shallow water about 50 feet offshore when the shark attacked, local media reports said. The woman's dog was also in the water.