Documents from the Department of Homeland Security detailing the response to exercises conducted in the event of a biological attack on Super Bowl Sunday were left on a commercial airplane and found by a CNN employee, according to the news outlet.

The documents, described as an “after action report” and marked “For Official Use Only,” were dated December 2017 and were related to two drills conducted to measure the coordinated response from public health, law enforcement, and emergency management officials if a biological attack were to occur in Minneapolis on Super Bowl Sunday.