Officials say swimmers at Coast Guard Beach in Massachusetts were evacuated after lifeguards spotted a pool of blood in the water.
Lifeguards flew a purple flag, indicating dangerous marine life, around 12:40 p.m. Thursday at the Eastham beach.
Leslie Reynolds, chief ranger at Cape Cod National Seashore, says beachgoers saw a group of seals swimming near the pool of blood. She says the blood might have been the result of a shark attacking a seal, but a shark was not spotted.
The beach remained closed for an hour.
A spokeswoman for the Atlantic Great White Shark Conservancy says there have been several brief closings due to shark sightings. On Aug. 12, beachgoers witnessed a great white shark bite a seal and spit it out onto the beach.