Makers of the EpiPen failed to investigate multiple complaints that the product malfunctioned during life-threatening emergencies, including some instances in which patients died, the U.S. Food and Drug Administration charged Thursday.

In a warning letter the FDA sent Meridian Medical Technologies Inc. on Tuesday and publicly announced Thursday, the agency told the company that it also failed to adequately respond after the manufacturer identified faults in some of its life-saving allergy auto-injectors. These faults included a failure to activate 171 times between 2014-17, according to user complaints, and the device’s failure to deliver the epinephrine in it when needed, the letter said.