The scream came from the ocean off the private Kukio Golf and Beach Club on Hawaii's largest island where, moments before, a man had been stand-up paddleboarding.
There's no lifeguard on the beach at the private community, but there is a "private safety team", which launched a four-person canoe.
In the water, they found "a male individual who had been bumped off his paddleboard about 90-140m offshore," Fire Captain Michael Grace told Honolulu Fox-affiliate KHON. "They recovered him from the ocean. He had injuries to his right-side extremities."
On the beach, bystanders and staff members tended to the man's wounds, applying multiple tourniquets before paramedics arrived, Grace said.
Authorities have not identified the man. They said he is a 25-year-old resident of the community at Kukio Golf and Beach Club and had been paddleboarding with his father.
Father and son told rescuers that a shark had bumped the younger man's board, then tore into him after he lost his balance and fell.
The injured man was flown to North Hawaii Community hospital with what were described as critical injuries in the first attack on the state's biggest island since 2015, according to the state's Department of Land and Natural Resources.
Between 2007 and 2016, 65 people were attacked by sharks in Hawaii, according to the International Shark Attack File. The state is a distant second to Florida for shark attacks in the US. Nineteen of those attacks happened near the state's biggest island, also called Hawaii.
Although shark populations are declining as the global population of humans grows, so does the number of beachgoers splashing into sharks' habitat.
As the International Shark Attack File wrote in a report about 2016 shark attacks:
"Shark populations are actually declining or holding at greatly reduced levels in many areas of the world as a result of overfishing and habitat loss, theoretically reducing the opportunity for these shark-human interactions.
"However, year-to-year variability in local meteorological, oceanographic, and socioeconomic conditions also significantly influences the local abundance of sharks and humans in the water and, therefore, the odds of encountering one another."
Most people who get attacked by sharks survive, nature writer Sy Montgomery told the Washington Post last year.
And people who get attacked are probably mistaken for something else by the sharks.
At the site of Saturday's attack, officials had posted signs warning visitors about the presence of sharks, according to KHON.
They were expected to decide today whether to reopen the beach.