Swimmers were ordered out of the water on Martha's Vineyard's South Beach after a reported shark sighting on Wednesday afternoon.
Lifeguards and swimmers said they spotted a fin off shore around 2 p.m.
Two shark sightings were reported on Wednesday within hours of each other, but it isn't clear if it might have been the same shark.
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Swimmers were allowed back into the water around 3 p.m. The U.S. Coast Guard is continuing to monitor the situation.
The Vineyard Gazette is reporting that the shark that was spotted was an 8-foot-long hammerhead.
Tony LaCasse of the New England Aquarium said hammerhead sharks are spotted off the Massachusetts coast every summer in small numbers.