A hammerhead shark sighting in the waters near Oceanside prompted lifeguards to close a stretch of the shoreline Tuesday.
Several people reported to lifeguards that they spotted the shark in the waters while standing on the Oceanside Pier Tuesday morning, Mason Turvey with Oceanside Fire Department Lifeguards said.
At about 10:30 a.m., a lifeguard at the end of the pier saw a shark, too.
The rare sighting prompted officials to shut down about two miles of the shoreline and post signs warning swimmers not to enter the water, from Harbor Drive to Wisconsin Avenue, for about four hours.
The beaches were reopened after lifeguards performed a search of the sea and could not spot the shark.
Turvey said it was unclear whether the sightings, which occurred at two different times of the day, were of the same shark or multiple.
The nine-foot hammerhead was last seen swimming about 100 yards off the coastline, lifeguards said.
Shark expert Dovi Kacev said the hammerhead shark species are unlikely to attack people unless provoked.
He said there have been recorded 17 hammerhead shark attacks, according to the international shark attack file run by the Florida Museum of Natural History.
"Most likely those are from Great Hammerheads, which we do not get locally," he said. "They are just the biggest of the group and have the largest prey (and mouths). The lack of a center notch suggests that is a smooth hammerhead, which is the species I would expect to see up here."
In July, a photographer told NBC 7 he captured video of a hammerhead shark near Mission Bay.
At the time, researchers with UC San Diego Scripps Institute of Oceanography said hammerhead sharks can sometimes be found in San Diego County waters in the warmer months.