The makers of the EpiPen failed to properly investigate more than 100 complaints that the device misfired during life-threatening emergencies — including situations in which patients later died, according to a Food and Drug Administration letter.

In a warning letter, the FDA said Meridian Medical Technologies, a Pfizer company that manufactures the auto-injectors used to treat life-threatening allergic reactions, had detected faults in some units of the EpiPen and failed to adequately respond — “including some situations in which patients subsequently died.”

The Sept. 5 warning letter noted that the manufacturers had received 171 complaints about products that failed to activate between 2014 and 2017, but did not disassemble “the vast majority” of the units as part of its investigation. The device is designed to inject epinephrine to halt dangerous allergic reactions.

“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 FDA letter said. Meridian Medical Technologies recalled certain lots of the injectors in March.

The FDA letter was sent to the Pfizer company, but the drug is marketed by Mylan, which has come under fire for the repeated price hikes it made to the price of EpiPen.

“Mylan has an unwavering commitment to quality and patient safety and we are confident in the safety and efficacy of EpiPen products being produced at the site. Further, we do not currently anticipate any supply issues as a result of the warning letter,” Mylan spokeswoman Nina Devlin said.