Officials at US Joint Base Andrews near Washington DC have given the "all clear" after reports of an active shooter situation sent the facility into lockdown earlier in the day.
Federal law enforcement officials said there was "not a shooter", and that the confusion stemmed from an active shooter exercise taking place at the base.
The official said someone in a different building saw the drill and called the police, and based on the call, authorities treated the perceived threat accordingly.
"The base was scheduled to conduct an active shooter exercise, however, reports of a real-world active shooter situation were reported," the base said earlier on Twitter.
Personnel at the base had been told to shelter in place on Thursday morning and emergency crews were sent to the scene.
Officials said the lockdown was done out of an abundance of caution.
The base is home to Air Force One and is often used by dignitaries travelling to the nation's capital.
Vice President Joe Biden was scheduled to fly out of the base later on Thursday morning for events in Ohio.
The US military has witnessed several shooting attacks on military bases in recent years. In July 2015, five service members were killed at two military offices in Chattanooga, Tennessee, and in 2009, gunmen at a US Army base in Fort Hood, Texas, killed 13 people.