Washington - US Joint Base Andrews outside Washington, the home base of the presidential plane Air Force One, was briefly placed on lockdown on Thursday amid reports of a shooter, but the military facility later gave the all-clear.The Washington Post, citing an unnamed federal law enforcement official, reported that there was "not a shooter".Confusion over a scheduled active shooter exercise at the Maryland facility was believed to have led to the lockdown, as someone called the emergency number 911 when they saw the drill, the Post said."The base was scheduled to conduct an active shooter exercise, however, reports of a real-world active shooter situation were reported," Joint Base Andrews said on Twitter, asking all personnel to shelter in place for about an hour.The base later tweeted: "'All Clear.' The lock down on base has been lifted."The situation delayed the travel of Vice President Joe Biden, who was due to head to Ohio for a campaign event, according to his office.Joint Base Andrews, located in Maryland, is home to thousands of military personnel and their families. US President Barack Obama regularly plays golf on the base.