Prosecutors in Lower Saxony state have opened an investigation after finding out about the tainted feed deliveries to those farms, Agriculture Minister Ilse Aigner said.
"This is a scandal within the scandal," she said.
Lower Saxony's agriculture ministry said products from those farms have likely been sold over the past ten days, "mostly eggs." But it reiterated its assessment that "consumption of these goods does not pose a health risk" given the low contamination level.
The scandal broke last week when investigators found excessive levels of dioxin in eggs and chickens, leading authorities to slaughter hundreds of animals and freeze sales from more than 5,000 farms. Excessive dioxin levels were also found in some pork.
As of Friday, all but 400 farms had been cleared and allowed to resume selling their products, but South Korea and China kept their bans on imports of German pork and poultry.
Aigner, who met with officials in Lower Saxony to discuss the dioxin scandal, appeared outraged by the state authorities' surprise announcement Saturday morning and urged Gov. David McAllister to immediately reprimand those responsible for failing to properly oversee the feed producer.
But a spokesman for the state's agriculture ministry, Gert Hahne, rejected Aigner's comments, saying authorities did their work properly and discovered the feed producer's wrongdoing. The firm itself was not named.
Authorities have traced the infected feed to Harles & Jentzsch GmbH, which is under investigation by prosecutors. The company, which filed for bankruptcy this week, is suspected of using tainted fat to make pellets that were then sold as livestock feed.