Washington: 'All clear' was sounded at the US Andrews Base, just outside of Washington, D.C., after a lockdown was ordered earlier due to "presence of an active shooter" reportedly barricaded inside the medical centre. Known as "America's airfield", Andrews is the home base of Air Force One, the airplane used by the US presidents. It is located in Maryland, four miles southeast of Washington, D.C. Vice-President Joe Biden was to fly out of Andrews at noon on Thursday, for a visit to Ohio. That trip was delayed until the situation is resolved, according to the White House. There appeared to be some confusion earlier over whether the "presence of the active shooter" was real or part of an exercise scheduled for Thursday. Base officials said the call came during an exercise, but the report "was real". "We are reacting to a report of a real-world active shooter" present at the base, a spokesperson for the base told WTTG TV. "The call occurred during an exercise. Because the report came in as a real active shooter, we have to respond accordingly. We are treating as if it is and no further information at this time." Homeland Security Secretary Jeh Johnson referred to an "unfolding situation" at Andrews in his Senate hearing. The situation was later described as normal and the lockdown was revoked. The most recent active shooter situation on an airbase was in April, when two Air Force members were killed in a murder-suicide at Lackland AFB in Texas.