Ernest Lungaro, director of enforcement at the Westchester County SPCA, said necropsies on three of the cats revealed blunt trauma to their skulls.
"Pretty disturbing, smashing their heads in and displaying them like that," he said. "We have found, in the past, cases where cats were poisoned, but we've never seen anything where they're killing them this violently."
Lungaro said a baseball bat, two shovels and a metal pipe were found near the scene in a wooded area just off Overlook Terrace in Yonkers, about a mile and a half north of the Bronx. He said investigators were not yet sure that those items had been used in the killings.
It also wasn't known if the killings were the work of one or more people, he said. Either way, they are disturbing because of studies that indicate a link between the killings of animals and violence against people, Lungaro said.
"The sheer number of cats that were killed with blunt trauma to the head, it's pretty violent," he said.
Some of the cats were just skeletons and some had been dead only three days, Lungaro said. The necropsies were done on the most recently killed animals.
The strange scene was discovered Thursday by a public works crew doing an annual cleanup. The SPCA was called in and counted 25 bodies, Lungaro said.
"We assume there were probably more than that because raccoons or whatever wildlife probably got to a couple of them," he said.
He said the necropsies suggest the cats were killed before they were put into bags.
Yonkers police and the SPCA are investigating. Lungaro said some people were being questioned but he would not say whether there was a suspect.
He said there are many feral cats in the area and there has been some tension over feeding stations that some residents have established.
"Some people get frustrated with the people who feed them," he said. He said it was possible the dead cats were put in the trees "to taunt the people that are feeding the cats."
A Maryland organization called Alley Cat Allies, which supports neutering programs for feral cats, offered a $750 reward for information leading to an arrest
Lungaro cautioned, however, that investigators had not yet established that the dead cats were feral. There has been no string of missing pets, he said, but "we are starting to get calls from people whose pets were lost."