The family of a Frederick County boy says the 5-year-old was bitten in the leg by a shark while vacationing in North Myrtle Beach, N.C. — but officials are less certain about what actually attacked the boy.

Isaac O’Hara, of Walkersville, Md., was bitten Saturday morning while playing in the water with his grandfather, the Sun News reported. The boy yelled out in pain, and his grandfather, Tom Gilbert, said he saw some fins headed back into the ocean.

Gilbert said he did not actually see a shark, and lifeguards did not witness the incident, said beach spokesman Pat Dowling.

Dowling said that the boy was definitely bitten by something, but no one knows for sure by what. Dowling said that bluefish could cause the same sort of bite.

Whatever bit O’Hara left a U-shaped wound about four inches long in his left upper thigh.