Authorities in New York have warned people visiting Prospect Park in Brooklyn to be on the lookout for an "unusually aggressive" squirrel.
Health officials said the "possibly rabid" rodent attacked at least five people at the park last week, four of whom have since been traced.
A fifth person, who was jogging when attacked, has been urged to seek medical attention, local media report.
Squirrels are rarely infected with rabies, a disease that can prove fatal.
But officials from New York's Health Department said in a statement they were assuming that this particular squirrel may have contracted the disease following reports of what it described as "unusual aggressive behaviour".
Anyone who thinks they may have been bitten in the park recently should get immediate medical advice, the department said.
However, regulars to Prospect Park are refusing to be intimidated by the animal.
"I'm not gonna sit there and be a victim of some squirrel," Tookes, a publicist from Crown Heights, told the New York Post. "Between me and my dog, we could take the squirrel," she said.
Another local, 36-year-old Sara Haynes, cast doubt on the rabies theory, suggesting that squirrels in Brooklyn were naturally aggressive, the New York Times reported. "They're just bold," she said, adding: "It's not stopping us."
The incidents occurred near the Parkside and Ocean Avenue entrance of the park between 18-20 July, local media report.
Rabies is a viral infection that affects the nervous system.
It is a zoonotic disease, one passed on to humans from animals, and is transmitted via saliva from infected animals - most commonly dogs.
There are no known cases in the US of squirrels passing the disease to humans.