Gillian Kocher, the director of public relations for the PSCPA, said they got a call on Saturday, April 18 that 13 puppies had been found abandoned in the area of Bread and Arch Streets.
However, by the time PSPCA's Animal Law Enforcement officers got to the area, they only found seven dogs.
"We suspect that perhaps some people may have taken the other puppies that were there in the meantime," said Kocher.
The PSPCA is now urging anyone who may have taken the missing puppies to return them to their Philadelphia headquarters as the dogs are evidence in an active animal cruelty investigation. They also say the safe return of the animals is critical.
The seven puppies that were found are healthy and currently in foster care, according to Kocher. She said they will eventually be put up for adoption.
Kocher said the Pennsylvania SPCA is now worried this case could be connected to another from December.
On the day after Christmas, another litter of puppies was found in the same area of Old City - in a parking garage near North Bread and Cherry Streets.
Those puppies, like the ones found this weekend, were estimated to be just three or four weeks old. Those six dogs were all eventually adopted.
"These cases seem eerily similar," Kocher told Action News. "They look very similar to each other, so we suspect they may be related. So we want to figure out sort of what's going on."
The Pennsylvania SPCA calls this a pattern of concern - with two abandoned litters discovered so close together, just months apart.
"Why are they being abandoned? Is there someone out there who needs our help? Are there animals that could be spayed or neutered?" Kocher asked.
The PSPCA is asking anyone with information about either litter, including where the puppies came from, who may have abandoned them, or the whereabouts of the six missing puppies from the most recent litter - to contact the organization's Animal Cruelty Hotline immediately at 866-601-7722 or cruelty@pspca.org. Tips can be left anonymously.