Officials continued their search on Monday for a swimmer who went missing near San Francisco in what may have been a shark attack.

A group of three swimmers ran into what they told officials was a shark off Wildcat Beach at Point Reyes National Seashore on Sunday morning.

The call came in just before 10:30 a.m., about 15 minutes after the missing swimmer had last been seen, Stinson Beach Fire Protection District Chief Jesse Peri told USA TODAY on Monday.

Witnesses reported seeing a shark, Peri said, but that has not yet been confirmed. Soon after, several other agencies including the National Park Service and the Cost Guard responded and launched a search.

Search for missing mom:'Let her come home': Family pleads for help finding missing Houston mom last seen leaving workplace

Coast Guard, National Park Service launch search

The Coast Guard's San Fransisco branch received a report of a person who appeared to have been pulled under the water by a shark, U.S. Coast Guard Petty Officer Hunter Schnabel told USA TODAY on Monday.

The man had been swimming with two others roughly 25 to 50 yards from shore when he was attacked, leaving behind a "large pool of blood in the water," reported KPIX-TV.

The responding agencies mounted a land, air and aquatic search that began Sunday afternoon and lasted overnight into Monday morning, Schabel said. After searching 62 square nautical miles over the course of 20 hours, the Coast Guard suspended the search as of 9 a.m. Monday pending any further updates.

No signs of the missing person or evidence as to what exactly occurred were found, said Schabel. While the witnesses reported seeing the shark again after the attack, officials are unable to officially confirm that such an attack occurred without laying eyes on the individual.

The National Park Service, Marin County Sheriff's Office, Marin County Fire Department and the Stinson Beach Fire Department are continuing a ground search.

Shark stuck on Florida shore:'It's too dangerous!' Massive mako shark stranded on Florida beach saved by swimmers

White sharks encounters

It's not uncommon for visitors to spot great white sharks at Point Reyes, specifically in late summer months through the fall. They most often hang out roughly 5 to 30 yards from shore, where their prey of harbor seals and California sea lions can be found, according to NPS.

Researchers found that great white appearances during peak seasons occurred as frequently as once every six hours at Point Reyes National Seashore, meaning a sighting is not necessarily rare on an average fall day. However, the likelihood of an attack remains "extremely small," according to NPS.

The identity of the missing swimmer was not revealed. The park service did not respond to requests for comment.