A southwestern Minnesota hen-slaughtering plant is the target of an undercover sting alleging inhumane treatment of animals, a claim the chicken company adamantly denies.

On Monday, the Humane Society of the United States unveiled results of its investigation of Butterfield Foods, a company about 45 miles southwest of Mankato that processes hens no longer useful for egg laying.

Many birds were ineffectively stunned before being killed, while others were still alive while being put into a tank of scalding water used in the de-feathering process, said Paul Shapiro, the Humane Society's vice president of farm animal protection.

"This is animal cruelty and cruelty perpetuated against a large number of animals on a daily basis," Shapiro said.

Terry Fruth, an attorney representing Butterfield, said the company adheres to standards for humane slaughter, and that on-site company and government observers ensure that.

"There are industry guidelines, and we are in compliance with all of them," Fruth said. "There are government rules, and we follow them."

Chickens in the United Startes are usually slaughtered by being shackled upside down, then stunned in an electrified water bath before having their throats cut. The birds bleed out, and are then desposited into a vat of scalding water to loosen feathers for plucking.

The Humane Society essentially claims that Butterfield's slaughter process allowed an inordinate number of birds to hit the scalding pot alive and conscious — an allegation Butterfield says is incorrect. Butterfield Foods is owned by Mankato-based Downs Food Group.

The Humane Society filed a complaint against Butterfield with the Watonwan County Sheriff's Office, alleging violations of Minnesota's animal cruelty law. The sheriff's office received the complaint Monday and has forwarded it to the Watonwan County attorney, who will decide whether prosecution is merited, said Rochelle Hanson, a sheriff's office investigator.

The Humane Society also filed a complaint with the U.S. Department of Agriculture.

The U.S. Humane Society's sting on Butterfield, which includes a video shot inside the plant, was undertaken by one of the organization's members who had worked at the plant for two months.

Such surreptitious operations by animal-rights groups have been heavily criticized by the agriculture industry, and are banned in some states.

In Minnesota in recent years, Christensen Farms and a Pipestone System farm — both major hog producers — have been targets of undercover investigations by animal rights groups. So has egg producer Sparboe Farms, which lost business with McDonald's and Target in the sting's aftermath.