The Humane Society of the United States released Wednesday a disturbing video and details of an investigation into meat safety.

The investigation found that sick and injured cows were still being processed, and the meat sent to a company that sends beef to Texas.

Graphic video attached to this story shows workers at a California slaughterhouse giving electric shocks to "downer" cows-- cows too weak or sick to stand on their own.

The six-month investigation was conducted by an undercover Humane Society worker at the Hallmark Meat Packing Co. in Chino, Calif.

Many of the practices seen on the videotape violate state and federal laws designed to keep unhealthy animals out of the food supply, for good reason.

Medical studies have found that downer cows have a higher risk of harboring spongiform encephalopathy, or mad cow disease.

In addition, doctors said there could be higher rates of E. coli and salmonella.

The Humane Society calls the investigation especially shocking because Hallmark Meat Packing and its processing partner, Westland Meat Co., supply meat to the National School Lunch Program.

The U.S. Department of Agriculture, which oversees the program, said it's looking into the incident.

In addition, in a statement released Wednesday, the two companies involved defended their safety procedures. They also said the two employees seen in the video have been fired.

We would like to warn you the video attached to this story contains disturbing images.