The FDA said Pfizer Inc’s process for manufacturing sterile injectable drugs at a plant in McPherson, Kansas was “out of control” and put patients at risk, according to a warning letter made public on Tuesday.

In the Feb 14 letter, the FDA said that several products were contaminated with multiple foreign particulates and that the injectable antibiotic vancomycin had been compromised by cardboard pieces.

It warned the company it had failed to take corrective action.

The presence of the foreign matter is “unacceptable” and suggests “a significant loss of control in your manufacturing process” that represents a “severe risk of harm to patients,” the FDA wrote.