Washington, June 30 (RHC)-- U.S. Joint Base Andrews, the military facility near Washington, DC that is home to the U.S. president's plane, was placed on lockdown on Thursday because of reports of a gunman at large and personnel were told to shelter in place.

Base officials issued an all-clear message for the base after about an hour, but said in Twitter messages that a medical center at the facility remained on lockdown.

A U.S. defense official said a second sweep was being conducted at the Malcolm Grow Medical Facility out of "an abundance of caution."  There had been no reports of gunfire or casualties.

Base officials said earlier that the military installation had been scheduled to conduct an active shooter exercise, but that there was a report of a "real-world" gunman at large at the medical center.

President Barack Obama, who flies out of Andrews on Air Force One, was not scheduled to use the base on Thursday.  U.S. Vice President Joe Biden was scheduled to depart from Andrews during the morning for a trip to Ohio.  He was reportedly told to "hold" at his official residence in Washington.