A great white shark broke free from a fisherman’s line and then swam off and savaged a long-distance swimmer.
The unnamed victim was taken to hospital with wounds to his stomach and chest in the attack off southern California’s Manhattan Beach.
One report said he is in a critical condition.
The incident began when a man fishing off Manhattan Beach Pier hooked the 10ft (3m) shark.
He spent 30 minutes trying to reel in the shark before the line broke.
Los Angeles County Fire Department said the same agitated shark attacked the victim about 40 minutes later.