The report stemmed from someone who made a distress call after seeing security forces doing a routine inspection yesterday.
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 20 miles 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.
"We take all threats seriously and reacted to ensure the security of those on the base," the statement read.
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Andrews is home to the presidential air fleet, including the planes that carry the call sign Air Force One when the president is aboard. 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 Wednesday night when he returned from a trip to Ottawa, Canada. Vice President Joe Biden was scheduled to leave from Andrews yesterday morning, but his trip was delayed by the lockdown.
"I think we need to pay attention to how to minimize 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.
"There was no catastrophic failure," Grollneck said. "Everybody took a pause, everybody evaluated what was going on and they started bouncing information off one another and realized there was no shooter," he said.