The ASPCA is investigating the deaths of four dogs within days of each other, all of whom were walked in Riverside Park in July.
All four dogs had seizures, foamed at the mouth and fell into comas during a two-day span, as first reported by DNAInfo.
A sign posted on a fence at the entrance to the park at West 137th Street warned dog owners of possible poison in the area.
All four dogs were walked in the same section of Riverside Park on July 17 and 18.
"I think it's being done deliberately," said Marc Calcano, who owns two dogs he's now keeping indoors out of fear they will get sick. "I think someone took it upon themselves to be malicious and hurt the dear and loving pets of many of our community members."
An ASPCA spokeswoman said the organization is investigating, but it has not determined whether the animals were poisoned, and if they were, whether it was deliberate.
While the investigation moves forward, dog owners in the neighborhood are being advised to keep an eye on their pets.
Veterinarian Dr. Kelson Danielson of Bluepearl Veterinary Specialists says if dogs are persistently sick with general malaise, lethargy, vomiting and diarrhea, owners should contact their vets.