Joint Base Andrews was locked down for nearly three hours Thursday after someone mistook a security exercise for an actual threat, officials said.
The Maryland Home of Air Force One was evacuated just after 9 a.m. because of a report of an active shooter. Personnel exited the building with their hands up, video shows.
Officials later determined there was no threat and declared an "all clear."
Officials said first responders received reports of an "real-world active shooter situation" about the same time the base was conducting a "no-notice" active shooter exercise. Officials said the drill was set to occur on the east side of the base. The mistaken threat was spotted on the other side of the base, in the medical center.
Someone on the third floor of the Malcolm Grow Medical Facility saw two people walking across the base with long guns, law enforcement officials told NBC News' Pete Williams. That person did not know a drill had been planned and reported that there was an active shooter in the building.
Reports of a real-world active shooter situation at the medical facility were "miscommunicated" before the drill began, Joint Base Andrews posted to Twitter after the incident.
U.S. Air Force Lt. Col. Ryan Freeland was shut out of the medical center building during the miscommunication. He said he was worried.
"Just praying for my colleagues who are in there barricaded behind locked doors," he said.
Defense Secretary Ash Carter said personnel need to take mistaken threats seriously but avoid calling in false alarms.
"I think it's important to have a reasonable level of awareness of the possibility of this kind of event and what to do. And I thought the response was strong and solid," he said. "So that's the good news. The bad news is it appears to have been a mistake, and we'd like to reduce the number of mistakes made in this way."
Col. Brad Hoagland, 11th Wing and Joint Base Andrews commander, praised first responders for their quick reaction.
"We take all threats seriously and reacted to ensure the security of those on the base," Hoagland said.
The lockdown was lifted and the investigation at the medical center continued. Rodney Smith, a patient advocate at the Malcolm Grow Medical Facility, told The Associated Press the situation unfolded at the newer of two buildings. Smith was in the older building.
Joint Base Andrews, in Prince George's County, Maryland, is the home of Air Force One and to other emergency reaction units for the area around the nation's capital.
Vice President Joe Biden was scheduled to fly out of the base Thursday morning for an event in Ohio, his office said. Biden was being held at the Naval Observatory during the lockdown.
Homeland Security Secretary Jeh Johnson addressed the investigation during his testimony at the Senate Judiciary Committee's hearing on oversight of the Department of Homeland Security. At the time, he called it an "unfolding situation" and said he would "take a break from this session" if need be.
Some military installations in the D.C. area increased their security in response to the investigation.
Fort Belvoir in Fairfax County, Virginia, said it increased its "security posture," and the Washington Navy Yard in Southwest Washington ordered "100 percent ID check" and "long guns at entrance gates." Security was also heightened at the Naval Observatory in Northwest Washington and the Naval Surface Warfare Center's Carderock Division in Montgomery County, Maryland.
Prince George's County — which would normally assist Joint Base Andrews during a security incident — said it did not assist with any emergency response.
Last month, Joint Base Andrews was placed on lockdown after a woman walked onto the base and claimed she had a bomb. An Explosive Ordinance Disposal team found the woman had no explosives, and she was apprehended.
The base has a long, storied history. The first prisoners of war back from Vietnam in 1973 arrived at Andrews Air Force Base as did the U.S. hostages from Iran in 1981.
Soviet Premier Nikita S. Khrushchev was the first foreign head of state to fly into Andrews in 1959.
Construction on a military airfield there began in 1942 under President Franklin D. Roosevelt. It was named Andrews Field in 1945 in honor of one of the founders of the Air Force, Lt. Gen. Frank M. Andrews. He had died in an airplane crash on May 3, 1943, the day the base opened.
The base's name was changed to Andrews Air Force Base in 1947, shortly after the Air Force became a separate service in 1947. It combined with the Naval Air Facility Washington to become Joint Base Andrews in 2009.