U.S. regulators warned Fresenius SE after the company’s Indian plant that makes cancer-drug ingredients for the U.S. market aborted hundreds of drug-quality tests because they seemed like they were going to fail due to impurities.

When workers at the plant found potential tainted products, they halted the tests and said human or machine errors were to blame instead, according to a Food and Drug Administration warning letter dated Dec. 4 that cited 248 aborted checks at the West Bengal facility.

“It is essential that you initiate an immediate and comprehensive assessment of your company’s global manufacturing operations to ensure that systems and processes, and ultimately, the products manufactured, conform to FDA requirements at all your sites,” the agency said in its letter, which was posted on Tuesday.

The agency recommended that Fresenius hire a consultant to help it improve its manufacturing practices. The company, based in Bad Homburg, Germany, didn’t respond to requests seeking comments.

The names of the ingredients involved were redacted in the FDA letter.

Fresenius is one Europe’s biggest provider of health care. According to documents known as “drug master files” filed in the past with the FDA, Fresenius manufactures ingredients used in chemotherapies to treat many kinds of cancer. The India plant cited in the letter is run by a unit called Fresenius Kabi, which has over 25 manufacturing facilities worldwide.

In its warning letter, the FDA asked Fresenius to review all failed quality tests that were invalidated for products that are meant to be sold in the U.S. and to evaluate all instances when a test was aborted and determine the potential effect on the quality of drug ingredients released for distribution.

The agency also warned that if the company doesn’t correct the issues raised in the letter, FDA workers could refuse products made at the facility admission into the U.S.