Regulators said the tainted milk was caught before it was sold in grocery stores or consumed.

But the source of the lindane still hasn't been found. To be safe, two state agencies continue to test milk at three northcentral Montana dairies. Tons of feed -- a likely source of the lindane -- has been embargoed at one dairy.

"There is no -- and I repeat no -- tolerance level in the regulations of EPA for lindane in milk that is going for human consumption," said Greg Murfitt, program manager for the state Department of Agriculture's pesticide enforcement program.

Lindane, an insecticide, is used to treat seed before it's planted to ward off soil-dwelling insects and as a deterrent to plant-eating bugs. In Montana, a special license is required to apply it.

Lindane-treated seed isn't supposed to be used as animal feed. The Extension Toxicology Network, a pesticide Web site supported by the USDA, calls it "highly toxic." Acute exposure in people can cause a variety of medical ailments.

"If you eat it, and eat it in a enough concentrations, it certainly isn't going to do us any good," Murfitt said.

On Oct. 13, lindane was found at .006 parts per million in a bulk tank unit at Meadow Gold Dairy in Great Falls, one of five Grade A milk processors in Montana. The tiny amount was too low to kick in FDA action.

"It just barley registered," said Ken Lee, milk and egg inspection bureau chief for the state Department of Livestock.

But the discovery prompted more testing by the state.

Bulk tanks contain milk from several dairies, so more samples were taken to pinpoint the offending dairy. That led to the New Miami Colony near Conrad.

State officials say New Miami is one of the cleanest in the state -- even the nation. "Us? No!" was the reaction of stunned colony members, who have been bringing milk to Meadow Gold for 24 years, colony secretary Jacob D. Wipf said.

"It didn't get to the shelf," Wipf said. "That's the most important thing."

Testing New Miami milk showed the level of lindane continuing to climb. When it reached the FDA action level of .30 parts per million Nov. 2, the state Department of Livestock barred the dairy from offering its milk for sale for human or animal consumption.

Wipf said the colony has no idea where the lindane originated.

After consulting with the FDA and the Center for Veterinary Medicine in Washington, D.C., Lee lifted the ban Nov. 19 when the level of lindane dropped to .1, well below the FDA's action level. The dairy's milk is being sold again and is perfectly safe, authorities said.

"At no time was public health in jeopardy according to FDA guidelines and regulations," said Karen Cooper, a spokeswoman for the Department of Livestock.

Lee, the milk and egg inspection bureau chief, said the contamination was caught shortly after it was introduced, a testimony to the checks and balances inherent in the state's milk-testing program. He said the cows did not become ill.

Statewide, milk is tested weekly for the presence of antibiotics, udder infection and bacteria in onsite labs at the processors. Insecticide tests are conducted twice a year. The Department of Livestock oversees the testing.

"I appreciate the state's effort on catching it and reacting so fast to it," said Billings-based Joe Bengoechea, general manager of Meadow Gold's Montana and Wyoming operations. "We were able to catch it and dispose of it."

The threat will persist, however, until the source of the lindane is found.

Through testing, the Department of Livestock tracked the insecticide to cattle feed. The Department of Agriculture, which regulates pesticide usage in Montana, was called in to do a "trace back."

Murfitt said the ag department is working with Canadian authorities, who are looking into a feed plant in Canada that was supplying feed to the New Miami colony and two other northcentral Montana dairies. With the investigation still in progress, he said he couldn't release the name or location of the Canadian feed supplier.

Four tons of feed have been embargoed at New Miami and can't be sold, fed to animals or disposed of until authorized by the state.

Murfitt is trying to figure out if the lindane was in the feed concentrate sent by the Canadian company, on the truck that hauled the feed or unknowingly introduced by the colony.

The Canadian company put together a recipe of feed for the colony, a common practice, Murfitt said. After dairies receive the feed concentrate, they typically add other feed constituents, such as hay, silage, barley and corn.

"It's all mixed together in a large feed wagon and goes to the cow," Murfitt said.

Testing at the two other Montana dairies that bought concentrate from the same Canadian company hasn't turned up any lindane, Murfitt said.

As of Wednesday, the most recent milk and feed samples still were being tested. Murfitt hopes to have the source figured out by the end of the month.

Under Montana law, misusing pesticide can result in license revocation and civil and criminal penalties.

"We're trying to find out the source and stop it," Murfitt said. "Then we'll decide whether or not there's any follow-up or ramifications."