Officials say lifeguards at a Massachusetts beach spotted a pool of blood in the water and ordered swimmers out.

Leslie Reynolds, chief ranger at Cape Cod National Seashore, says swimmers at Coast Guard Beach in Eastham saw a group of seals near the blood. She says it might have been the result of a shark attacking a seal, but a shark was not spotted.

Lifeguards flew a purple flag, indicating dangerous marine life, around 12:40 p.m. Thursday. 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.