U.S. Joint Base Andrews , the military facility near Washington that is home to the 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 now holding at his official residence in Washington, his spokeswoman said.
"First responders are on-scene now," base officials said on Twitter. "Due to the serious nature of this report, the base is reacting to ensure the safety of all personnel."
No other details were immediately available, and base officials did not return calls for comment. Prince George's County police directed questions to the base.
The U.S. military has been on high alert for possible attacks at U.S. locations after incidents such as the July 2015 shooting rampage that killed five service members at two military offices in Chattanooga, Tennessee, and the 2009 shooting at a U.S. Army base in Fort Hood, Texas, in which a gunman killed 13 people.
Homeland Security Secretary Jeh Johnson, asked about the Andrews incident during testimony before a Senate committee, said there was an unfolding situation at the base and that he might have to leave the hearing at some point to monitor the situation.
The base in Maryland is the primary military air installation in the Washington, D.C., area, and is located about 15 miles (24 km) from the White House.
Malcolm Grow is a full-service military hospital which according to its website is affiliated with the Uniformed Services University of Health Sciences and serves as a training facility for 48 medical residents and 31 health professionals.
In May, Joint Base Andrews was put on lockdown after a woman claiming to have a bomb strapped to her chest arrived at the visitor center. The woman was apprehended and an explosives team determined there was no bomb.