Filed at 3:46 p.m. ET

MINNEAPOLIS (AP) -- The discovery of low levels of dioxin in livestock feed ingredients from a Minnesota plant has prompted a nationwide investigation into whether other companies might be making feed supplements containing the carcinogen.

The findings could provide insight into how trace levels of dioxin find their way into meat and poultry.

The dioxin levels found in the feed supplements made at Chaska-based Quali Tech were so low, especially after they were diluted in feed, that they wouldn't pose a health hazard to people, said Hillary Carpenter, a toxicologist with the Minnesota Department of Health.

But Carpenter and Food and Drug Administration regulators are questioning whether the contamination could be an industrywide problem.

``If it is process-related, it could have been going on for some time,'' said Greg Buzicky, director of agronomy and plant protection at the Minnesota Department of Agriculture. ``Until a few years ago, we didn't have the technology to look at those low levels.''

Since 1994, the Environmental Protection Agency has sought to reduce dioxin levels in the environment, in meat and in people.

The American Feed Industry Association in Arlington, Va., is urging companies to cooperate with investigators, spokesman Rex Runyon said.

The problem at Quali Tech was found through a random sampling by Irish officials of feed that the company exported. On Feb. 27, Quali Tech officials reported to the FDA that an Irish firm had found dioxin in its products. The probe accelerated this week after preliminary tests indicated dioxin had apparently been produced during manufacturing.

Follow-up tests ordered by Quali Tech found low levels of dioxin in a feed ingredient that contained powdered copper. Health and agriculture officials said they are uncertain how the dioxin formed, but one theory is that chlorine in the city water used by the plant contributed to the production of dioxin during a heating process.

The family-owned company is temporarily shut down and revising its production process, said attorney Jim O'Neal.

Quali Tech issued a voluntary recall on Friday for its feed supplement ingredients, which are sold under such names as SQM Mineral Products and Carbosan Mineral Products.