Charges will not be filed against workers at a Minnesota hog farm who were recorded on a hidden camera treating animals in a way that their own employer called "disturbing," officials announced Friday.

The Los Angeles-based watchdog group Last Chance for Animals released grisly footage in August that appeared to show sick sows and piglets being abused and neglected at a Christensen Farms breeding facility in Luverne, Minn. The company, one of the nation's largest pork producers that supplies national chains like Wal-Mart, responded this summer by suspending seven employees and launching an internal investigation.

Assistant Rock County Attorney Jeffrey Haubrich said Friday that his office would not file animal cruelty charges against any employees, as the watchdog group had demanded. In a letter to Sgt. Jeff Wienecke of the Rock County Sheriff's Office, which was obtained by the Star Tribune, Haubrich said that the videos are not admissible in court and that an outside veterinarian following up at the farm failed to find any evidence that could lead to criminal charges.

Haubrich explained his decision by citing "substantial evidentiary issues with the material" provided by Last Chances for Animals.

"The video and reports are obviously highly edited and filtered to enhance the position they are advocating and they lack the basic requirements for admissibility in court," he wrote. "Namely, there is a lack of foundation and no chain of custody for the main pieces of the evidence that have been presented."

An outside veterinarian deemed the facility's methods "acceptable within industry standards" and said the "animals appeared well cared for," Haubrich wrote.

Star Tribune, Star Tribune
The Los Angeles-based watchdog group Last Chance for Animals released grisly footage in August that appeared to show sick sows and piglets being abused and neglected at a Christensen Farms breeding facility in Luverne, Minn.

Adam Wilson, director of investigations of Last Chance for Animals, said none of the reasons Haubrich cited for failing to charge employees were valid. Wilson said the group presented law enforcement many hours' worth of their unedited original recordings and also offered to make their undercover witness available to corroborate its authenticity. But Wilson said the tapes were never reviewed and the witness never questioned by authorities.

"I don't consider that a proper investigation was conducted," he said. "It really highlights the fact that this was a political decision made not to go after Christensen Farms as a Minnesota mega-farming business."

After its four-month investigation, the group released video footage to the public depicting sick and wounded pigs languishing for weeks in the company's "sick pens"; being dragged by the ears and snouts; and being slapped and stabbed with ink pens to force them to walk.

The Last Chance for Animals video is the second undercover operation by an animal rights group at Christensen Farms in the past three years. In 2012, Mercy for Animals shot footage of hogs in cramped gestation crates, essentially cages in which sows live. It's a common pig housing system.

Christensen Farms officials could not be immediately reached for comment Friday night. It remains unclear whether the suspended workers still work for the Sleepy Eye-based company.

The Associated Press contributed to this report.

Liz Sawyer • 612-673-4648