After an eight-month investigation, the Humane Society of the United States accused Petland, the national pet store chain, of selling dogs bred under appalling conditions at puppy mills around the country.

The animal protection group made the charges at a news conference in Washington Thursday. The investigation involved 21 Petland stores and dozens of breeders and brokers. The Petland stores are being supplied by large-scale puppy mills, although customers are routinely informed that the dogs come only from good breeders, the Humane Society said.

"They are buying from puppy mills where these dogs are not treated like pets," Michael Markarian, an executive vice president with the Humane Society, told a news conference. "They're treated like a cash crop, where mother dogs live in wire cages, sometimes stacked on top of each other in filthy, dirty, cramped conditions, where they receive little socialization or human interaction or exercise."

Dogs from puppy mills are sold at Petland stores for as much as $3,500 each, according to the Humane Society.

Investigators reviewed interstate import records of an additional 322 breeders, U.S. Department of Agriculture reports and more than 17,000 individual puppies linked to Petland stores, according to a release on the group's Web site.

Filthy cages, inadequate care

Among the poor conditions cited, investigators found puppies in commercial breeders "living in filthy cages reeking of urine, with inadequate care and socialization," according to the release. The Humane Society says dogs at the mills were found in cages with wire flooring so large that the puppies' paws and even the paws of the mother dogs would fall through.

The group said pet stores should not be buying puppies from "abusive puppy mills" and "should not be lying to consumers" about where they get their puppies.

A call to Petland corporate offices in Chillicothe, Ohio, was not immediately returned. In a statement, Petland said the company does not support substandard breeding facilities and provides each store with guidelines on humane care of animals.

A statement on the company's Web site noted that "Petland stores are independently operated by qualified franchisees. Each is responsible for choosing healthy pets offered to Petland customers. Petland, Inc. provides each Petland store with humane care guidelines to assist in this important task."

Individual Petland stores previously have been targeted by animal rights activists for reselling puppies supplied by commercial breeders.

Large commercial breeders are legal and regulated by the USDA, but enforcement of humane conditions is a low priority , according to a recent report on msnbc.com.

The Humane Society investigation comes as legislators recently have stepped up moves to crack down on the lucrative puppy mill industry. In October, Pennsylvania Gov. Ed Rendell signed a bill imposing stricter standards on commercial kennels , including regular veterinary exams, larger cages and exercise areas. At least three other states have issued laws this year placing restrictions on commercial breeders.

More on puppies | pet health