Washington Heights dog walkers, beware: four dogs have died soon after being walked in Riverside Park this summer, and now owners believe they were poisoned, prompting an investigation by the ASPCA.

The dogs were all walked in the area on July 17 or 18, and suffered fatal seizures a few hours after being in the park. Kim Heismann, whose dog Charlie was one of the canine victims, told CBS 2 that her roommate's dog died on the same day as her own, also hours after being walked in the park. Also on that same day, neighbor Matthew Wood's French bulldog Tugs suffered a seizure and died less than 12 hours after walking in Riverside Park, and Wood told the Post that another woman on his and Heismann's block lost her dog in the same fashion.

Benjamin Davidson, a critical care veterinarian at the BluePearl clinic where Heismann's dog was taken before he died, says the seizures leading to the Heismann and her roommates' dogs' deaths suggest "toxin ingestion." "It's too coincidental," Davidson told DNAinfo. "If two other dogs died and showed similar signs such as a seizure, I would be concerned that someone put something down." And while rat poison might seem like the obvious culprit, the Parks Department says they stopped such poisoning well before the dogs died, and Dr. David Bessler of Bluepearl says the dogs' symptoms don't fall in line with those caused by rat poison. "The majority of rat poisons that we see out there cause bleeding and not seizures like this," Bessler told CBS 2.

Heismann has alerted the ASPCA about the deaths, and they are investigating whether or not someone has been poisoning the dogs on purpose. Heismann told DNAinfo she received an anonymous call from a resident on 137th St. who said his superintendent had been putting down poison in retaliation against people who didn't pick up after their dogs. The ASPCA says such an act could warrant a criminal charge of animal cruelty. Earlier this year, a number of hawks died in city parks, and officials suspected they were poisoned.