If you review the raw data documentation and any accompanying research papers, you will be informed that "event_key" is meant to be an important field.
Master Event Data: A distinct master event data record will be present for each source reporting anevent. In other words, if a User Facility, Distributor, Manufacturer, and voluntary submitter all report an event, there will be four event records. These individual source records are related via the EVENT KEY. EVENT KEY is an internally-generated key which links multiple sources to a single event.
However from both the API queries and text file downloads the event_key field seems to be completely empty. Documentation also has missing details to help answer the question. Hope to get a response on this!