PRINCE GEORGE CO, MD (WCMH) — Joint Base Andrews is no longer on lockdown following a report of an active shooter at the Malcolm Grow Medical Facility.

Senior officials now tell NBC News the situation at JBA was a misunderstanding during an active shooter drill. The all clear was given at the base just after 10:30am local time.

The Air Force Base sent a tweet Thursday morning instructing all personnel at the base in Washington’s Maryland suburbs to shelter in place. It says the call came in just after 9am local time.

A second tweet confirmed the incident was taking place at the Malcolm Grow Medical Facility. It instructed everyone to continue to shelter in place, a precaution meant to keep people safe while remaining indoors.

Senior officials later told NBC News that the base was scheduled to conduct an active shooter exercise today. While setting up for the drill, two personnel were walking across the base carrying long rifles.

Someone in the medical center then saw those guns and phoned in that there was a shooter on the base. The reports of a real-world active shooter situation then prompted the lockdown.

Joint Base Andrews is located about 20 miles outside of downtown Washington, D.C. and hosts aircraft used by President Obama including Air Force One.The Associated Press contributed to this report.Keep checking NBC4i.com for real-time updates on this story. To get alerts for breaking news, grab the free NBC4 News App for iPhone or Android. You can also sign up for email alerts.What others are clicking on: