The Food and Drug Administration described “significant violations of current good manufacturing practice” in a warning letter to a Pfizer Inc. unit that manufactures Mylan NV’s EpiPen allergic reaction treatment.

The Pfizer PFE, -0.51% unit, Meridian Medical Technologies Inc., “failed to thoroughly investigate multiple serious component and product failures... including failures associated with patient deaths and severe illness,” the warning letter said, referring to an FDA inspection of a Missouri manufacturing facility between February and March of this year.

“You also failed to expand the scope of your investigations into these serious and life-threatening failures or take appropriate corrective actions, until FDA’s inspection,” said the letter.

See: Mylan stock slumps 1.3% after company says voluntary recall will include U.S. EpiPens

In February 2016, Meridian found that one unit of an EpiPen component that ensures the product delivers its dose of epinephrine did not work. The company rejected that lot and another one, but the company continued to manufacture its products while the investigation continued through October 2016, according to the FDA letter.

Meanwhile, Meridian received hundreds of complaints about EpiPens failing to operate in emergencies, “including some situations in which patients subsequently died,” with many of the complaints having to do with dose delivery issues, said the FDA letter, which was dated Tuesday and released publicly on Thursday.

The company did not thoroughly investigate the complaints, nor did it remove “potentially defective products from the marketplace, even though you had identified a defect in one of the critical components used to manufacture the products and even though you ultimately confirmed the same or similar component defect as the root cause for multiple complaints,” the FDA said.

A May 2016 investigation into a customer complaint found the same manufacturing defect from February, the FDA said, but the company concluded the defect was infrequent, did not link the two issues and decided not to take market action.

The FDA also took issue with how the company investigated complaints.

Of 171 complaint samples between 2014 and 2017 for products that did not activate correctly, Meridian disassembled and examined very few, the FDA said.

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At the FDA’s prompting, Meridian reopened an investigation it had closed in June 2016. But the company only recalled EpiPens after the FDA’s inspection closed and “after multiple discussions with FDA,” the FDA said.

Thirteen of the recalled lots were distributed in the U.S.; all were recalled in March. A Wall Street analyst estimated at the time that the lots consisted of about 260,000 devices.

Mylan MYL, -2.66% emphasized at the time that the recall was a “precautionary measure,” citing reports of two EpiPens outside of the U.S. failing to activate. There was a possibility that the recalled lots contained a defective part and might not work in an emergency situation, Mylan said.

Read: Here’s how much Mylan’s U.S. EpiPen recall could cost it

The FDA also criticized Meridian’s procedure for handling complaints, its systems for identifying existing and potential quality problems and its ability to verify that designs meet its specifications.

The regulator asked the company to review its manufacturing investigations and provide its plan for addressing safety risks, all within 15 days.

If the company does not promptly fix the violations, there may be legal action, and the FDA may not approve applications listing the manufacturing facility, the FDA said.

Mylan came under pressure last year for hiking the price of the EpiPen sixfold in the last few years. Mylan acquired the product in 2007 and the price increases since then were among the biggest of any top-selling brand drug. Mylan Chief Executive Heather Bresch was forced to testify to a congressional committee, where she was excoriated for the company’s pricing decisions.