A report of an "active shooter" at a US military base outside Washington where the presidential plane Air Force One is stationed turned out to be a false alarm, though it locked down the base for more than an hour.
The report stemmed from someone who made a distress call after seeing security forces doing a routine inspection on Thursday.
The confusion was heightened by a planned active shooter drill at Joint Base Andrews that had not yet begun. Officials said in a Facebook post that there was no shooter and no threat to the base or workers there.
The base, about 32km from Washington, was placed on lockdown around 9am. About an hour and a half later, the military post tweeted that the lockdown had been lifted, except for the medical building where the active shooter was reported.
In a later statement, the base confirmed there was no gunman and no threat to public safety.
"Fortunately, this was not a life-threatening situation," Colonel Brad Hoagland, 11th Wing and base commander, said in the statement. "We take all threats seriously and reacted to ensure the security of those on the base."
Andrews is home to the presidential air fleet, including the planes that carry the call sign Air Force One when the president is on board. The president, vice president and other senior government officials fly in and out of Joint Base Andrews.
President Barack Obama was last at the base on Wednesday night when he returned from a trip to Ottawa, Canada.
Vice President Joe Biden was scheduled to leave from Andrews on Thursday morning, but his trip was delayed by the lockdown.
Defense Secretary Ash Carter said the situation was handled relatively well despite the apparent communication problem that led to the false report.
"I think we need to pay attention to how to minimise the chances of false alarms like that," Carter said.
"At the same time, I think it's important to have a reasonable level of awareness of the possibility of this kind of event and what to do, and I thought the response was strong and solid."
Chris Grollneck, an active-shooter prevention consultant who has worked on training at Army and Air Force bases, said the response to the report at Andrews was well-orchestrated and shows how much the military's training for active-shooter situations has improved. He also said the person who reported the shooter should be praised for taking the "see something, say something" message seriously.
