FRESNO, Calif. - Federal regulators who shut down a Central California slaughterhouse after receiving an animal welfare video were investigating Tuesday whether beef from sick cows reached the human food supply.

The video appears to show workers bungling the slaughter of cows struggling to walk and even stand. Under federal regulations, sick animals cannot be slaughtered for human consumption.

The investigation will determine whether sick cows were slaughtered and whether meat products from the company should be recalled, a spokesman for the USDA Food Safety Inspection Service said.

The agency suspended operations Monday at Central Valley Meat Co. in Hanford after receiving the video Friday from the animal welfare group Compassion Over Killing. The footage shows animals bleeding and thrashing after being repeatedly shot in the head with a pneumatic gun in unsuccessful efforts to render them unconscious for slaughter.

Federal regulations say that to avoid unnecessary suffering during slaughter, animals must be rendered unconscious by a single shot to the head from a pneumatic gun that fires a bolt through the skull to pierce the brain.

The USDA said late Tuesday that it found evidence of inhumane practices on the video, but the footage did not indicate beef from sick cattle got into the food system. However, the investigation is continuing.

"We have not substantiated a food safety violation at this time. We are aggressively continuing to investigate the allegations," said Al Almanza, administrator of the Food Safety and Inspection Service.

In-N-Out Burger, a fast food chain, severed its ties with the company after learning about the situation. Mark Taylor, chief operating officer, said Tuesday the company acted immediately upon becoming aware of it.

"In-N-Out Burger would never condone the inhumane treatment of animals and all of our suppliers must agree to abide by our strict standards for the humane treatment of cattle," Taylor said to The Associated Press in a written statement.

In-N-Out's agreement with suppliers also prohibits companies from shipping beef from sick animals.

Central Valley Meat Co., owned by Brian and Lawrence Coelho, declined to comment on the video, saying company officials had not seen it. On Tuesday, the company hired a public relations firm that issued a statement saying Central Valley Meat Co. is cooperating with investigators.

"Central Valley Meat takes these issues very seriously and is now developing a plan of action to present to (the Food Safety Inspection Service) to remedy any potential violations of USDA guidelines," the statement said. "Based on our own investigation and 30 years of producing safe, high-quality US beef, we are confident these concerns pose no food safety issues."

The video taken by an undercover investigator for Compassion Over Killing shows cattle lying in pens unable to move, and at least one unable to stand to leave a stock transportation trailer.

Some clips show cattle with swollen udders that are unable to keep their legs under them. Other footage shows a downed cow trembling and unable to stand even as workers try to pull her up by the tail.

Within hours of seeing the video, the USDA's Office of Inspector General sent investigators who found evidence of "egregious inhumane handling and treatment of livestock." The possibility that animals were being inhumanely treated caused officials to shut down the plant while the investigation unfolds.

The USDA had at least two inspectors stationed at the site, and federal officials, when asked whether there was evidence the inspectors had neglected their duties, said the investigation is ongoing.

The USDA received hours of videotape from the Washington D.C.-based animal welfare group, which said its undercover investigator was employed by the slaughterhouse and made the video over a two-week period in June and early July.

Four minutes of excerpts the animal welfare group provided to The Associated Press showed cows being prepared for slaughter. One worker appears to be suffocating a cow by standing on its muzzle after a gun that injects a bolt into the animal's head had failed to render it unconscious.

In another clip, a cow is still conscious and flailing as a conveyor lifts it by one leg for transport to an area where the animals' throats are slit for blood draining.

"The horror caught on camera is sickening," said Erica Meier, executive director of Compassion Over Killing. "It's alarming that this is not only a USDA-inspected facility but a supplier to the USDA."

Online USDA records show the company has contracted to sell ground beef to USDA food programs.

"It's a good sign that the USDA is taking this seriously, but I want to see what comes next," said Meier of Compassion Over Killing, adding the video will be posted on the organization's website.

The case is reminiscent of a 2008 undercover operation by the Humane Society of the United States at the Hallmark slaughter plant in Chino that led to the largest-ever recall of beef and the conviction of two people found to have treated cows cruelly. In that case, video showed downed cows being prodded with a folk lift.

Tracie Cone can be reached at http://www.twitter.com/TConeAP