The manufacturer of EpiPen devices failed to address known malfunctions in its epinephrine auto-injectors even as hundreds of customer complaints rolled in and failures were linked to deaths, according to the Food and Drug Administration.

The damning allegations came to light today when the FDA posted a warning letter it sent September 5 to the manufacturer, Meridian Medical Technologies, Inc. The company (which is owned by Pfizer) produces EpiPens for Mylan , which owns the devices and is notorious for dramatically raising prices by more than 400 percent in recent years.

The auto-injectors are designed to be used during life-threatening allergic reactions to provide a quick shot of epinephrine. If they fail to fire, people experiencing a reaction can die or suffer serious illnesses. According to the FDA, that’s exactly what happened for hundreds of customers.

In the letter, the agency wrote:

In fact, your own data show that you received hundreds of complaints that your EpiPen products failed to operate during life-threatening emergencies, including some situations in which patients subsequently died.

The agency goes on to lambast Meridian Medical for failing to investigate problems with the devices, recall bad batches, and follow-up on problems found. For instance, a customer made a complaint in April 2016 that an EpiPen failed. When Meridian disassembled the device, it found a deformed component that led to the problem—the exact same defect it had found in February when another unit failed.

The FDA wrote:

Nonetheless, on June 3, 2016, you concluded that the defect was infrequent, even though you had not examined all of your reserve samples to determine the extent of the defect within the same lot of finished products, nor did you expand your investigation to other lots... You closed your investigation and determined that “no market action would be taken.”

The agency went on to note that, during an inspection earlier this year, Meridian employees said that they did not disassemble the “vast majority” of failed devices customers sent back to them. Meridian received 171 such “complaint samples” between 2014 and 2017. But employees said they were only allowed to take devices apart if it was “approved by management.” Yet Meridian noted that disassembly was the only way to determine why a device failed.

Days after the inspection, Mylan recalled tens of thousands of injectors.

In a statement today, Pfizer responded to the FDA’s letter, saying:

Patient safety is of primary importance to Pfizer. We stand behind the quality, safety, and efficacy of the products we manufacture. We will continue to work with the FDA to resolve the points raised in the letter.

A spokesperson for Mylan told CNBC: “Pfizer is continuing to work with FDA to resolve the points raised in the letter regarding Pfizer’s manufacturing of EpiPen Auto-Injector, and Mylan will do whatever it can to support this process.” The company did not expect any EpiPen supplies due to the letter.