In all instances, the Humane Society said, the treatment amounted to animal abuse. The group said they could not determine if the animals ever entered the food supply.

Unlike slaughter plants that are overseen by inspectors from the US Agriculture Department, there is no regulatory presence required from state or federal officials at interstate markets such as auction houses, the group said.

"There is a gap in the regulatory process. No one is watching. No one is taking responsibility for these animals," Wayne Pacelle, president of the Humane Society of the United States, told reporters.

The evidence was presented to Agriculture Secretary Ed Schafer last week. Pacelle said the group urged the USDA to close a "loophole" that allows downed cattle to be slaughtered in rare cases while expediting rules to require more humane treatment of animals.

"Since learning about this investigation, we are reaching out to states and industry groups to address this issue," said Schafer. "In my conversation with the Humane Society last week, I expressed my sincere desire to work with them."

Beef from downer cattle -- defined as an animal too ill or injured to walk -- is generally not allowed in the food supply. The rule was adopted as a safeguard against "mad cow" and food-borne diseases. Meat packers are required to alert USDA veterinarians of downer cattle so they can decide if the animal can be slaughtered for food.

At the Clovis Livestock Auction in New Mexico, two downed cattle were filmed by the Humane Society over five hours April 30, including one animal that was flailing her legs as she expelled feces into the pen where other cows were held.

Charlie Rogers, owner of the facility, said his business does not allow non-ambulatory animals to be unloaded, and will euthanase sick or injured animals as soon as possible. In this case, the health of the animals deteriorated after they arrived.

"I've been here 22 years and I've never had a complaint against me before this," said Rogers. "I'm just as concerned about proper handling of livestock as anybody. That's how I make a living."

Randy Bouldin at the Livestock Exchange in Hereford, Texas, said he wishes the Humane Society would have told him about the two downed cows on his property April 1 so they could have been humanely euthanased immediately.

"I feel like the Humane Society's primary objective was to get a story rather than the concern of the two cows," he said.

The findings from the livestock auctions were the second investigation made public by the Humane Society this year.

A videotape released on Jan. 30 showed Hallmark/Westland Meat Packing Co workers using abusive techniques to force sick and injured cattle into a slaughterhouse so they could be processed into food.

The practices at the California Hallmark plant triggered the recall of almost 143 million pounds (65 million kg) of meat, mostly beef. It was the largest recall in US history.