US military Joint Base Andrews, just outside of Washington, DC, was put on lockdown due to an active shooter reportedly barricaded inside the medical center. A "shelter in place" order went into effect at 9 a.m. local time.

Known as "America's airfield," Andrews is the home base of Air Force One, the airplane used by US presidents. It is located in Maryland, four miles southeast of Washington, DC.

Vice-President Joe Biden was supposed to fly out of Andrews at noon on Thursday, for a visit to Ohio. That trip has been delayed until the situation is resolved, according to the White House.

JBA is currently on lockdown due to a report of an active shooter. All personnel are directed to shelter in place. More info as it comes. — Joint Base Andrews (@JBA_NAFW) June 30, 2016

UPDATE: FBI and Maryland State Police confirmed that the situation was a scheduled drill. An all-clear has been given as of 10:30 a.m.

"All Clear" for the base with the exception of Malcolm Grow medical facility — Joint Base Andrews (@JBA_NAFW) June 30, 2016

The incident was reportedly unfolding at the Malcolm Grow Medical Facility, the base authorities said. There were no reports of actual shots fired, however.

The incident is ongoing at the Malcolm Grow Medical Facility. First responders are on-scene now. All personnel continue to shelter in place — Joint Base Andrews (@JBA_NAFW) June 30, 2016

The shooter was barricaded on the third floor of the medical center, according to a reporter for the military newspaper Stars and Stripes.

Active shooter is barricaded into third-floor of medical facility building at Joint Base Andrews, personnel sheltering in place — Travis J. Tritten (@Travis_Tritten) June 30, 2016

Another report also said the situation was unfolding at the medical center, citing the Air Force.

Heads up from the AF Service Watch Cell. Received report of an Active Shooter at Joint Base Andrews, Bldg 1060, Medical Facility. — ImJff (@JffThchr) June 30, 2016

There appeared to be some confusion whether the situation was real or part of an exercise scheduled for today. Base officials said the call came during a drill, but they were treating the report as real.

“We are reacting to a report of a real world active shooter,” a spokesperson for the base told WTTG. “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.

At a Senate hearing, DHS Secy Jeh Johnson said "unfolding situation at Andrews Air Base" might require he "take a break" from the hearing. — Mark Knoller (@markknoller) June 30, 2016

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.