The incident aboard the flight to Munich occurred in June.
"A United pilot improperly disposed of ammunition during a flight last month from Houston to Munich," the airline said in a statement Thursday. "We are reviewing the incident, and the pilot remains an employee."
Under federal law, a pilot is allowed to carry weapons, but only in the United States.
"Under the Federal Flight Deck Officer program, eligible flight crew members are authorized by the Transportation Security Administration to use firearms to defend against an act of criminal violence or air piracy attempting to gain control of an aircraft," TSA said.
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