Beachgoers on Florida's Gulf Coast have been told to be vigilant after sharks injured three swimmers in two separate attacks.
A woman had part of her arm amputated after Friday's attack in Walton County in northwest Florida.
Less than two hours later, on another beach six kilometers to the east, two teenage girls were in the shallows when they were attacked.
One of the girls suffered "serious injuries to her upper leg and one arm," and the other suffered minor injuries to her feet, rescue services said.
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Services are patrolling the coast, and some beaches were temporarily closed.
The day after the attack, the beaches were reopened, but a purple flag was displayed on them as a warning to bathers that there is danger lurking in the water.
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The first incident happened on Friday around 13:20 local time when a woman, around 45 years old, was attacked near Watersound Beach, authorities said.
She suffered "critical injuries" to her hip and lower left arm and was taken to hospital, firefighters said.
Part of her hand had to be amputated, journalists were told afterwards.
The second attack on two 15-year-old girls was recorded around 14:55 p.m. local time near Seacrest Beach, authorities added.
"Swim carefully, respect the Gulf and look after your loved ones," read the warning from the authorities.
The Walton County Sheriff's Office said on social media Saturday that a 4,2-foot-long hammerhead shark had been spotted off Santa Rosa Beach during patrols, but stressed that they are "not unusual."
"We want to reiterate that sharks are always present in the Gulf," they said.
"Swimmers and beachgoers should exercise caution when swimming and be aware of their surroundings."
Prema University of Florida International Shark Attack File, every year around 70 to 100 shark attacks occur around the world, and around five cases end tragically.
Last year there were 69 unprovoked shark attacks on humans and 22 worldwide.
In Florida, sharks from warm-sea species, including bull sharks and blacktip sharks, are responsible for most attacks.
Most attacks are in coastal waters, usually near sandbars where sharks feed and can become trapped at low tide.
Small fish move in schools near shore this time of year, which may have contributed to Friday's attacks, the Bay County Sheriff's Office suggested.
The timing of the two attacks -- early afternoon -- was unusual, Walton County Sheriff Michael Adkinson said.
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