Joint Base Andrews Lockdown Lifted

Joint Base Andrews in Prince George's County, Maryland, was on lockdown for over one hour this morning after someone at the base's Malcolm Grow Medical Clinic saw guns and called authorities, a law enforcement source said. The reported activity turned out to be a security team doing routine work, a spokesperson told ABC News.

The lockdown has since been lifted.

The base explained that a "no-notice active shooter exercise" was scheduled for the late-morning. Capt. Connie Dillon, chief of public affairs, told ABC News that only senior leadership was informed about the active-shooter drill.

But before the drill occurred, someone called in suspicious activity near the medical clinic - which is on the opposite side of the base from where the drill was planned.

The reported suspicious activity turned out to be a uniformed base security team doing routine work, Dillon said.

The lockdown was reported at about 9:15 a.m. ET. First responders arrived at the scene and all personnel were told to shelter in place. The all-clear was given at 10:40 a.m., Dillon said.

Local law enforcement was on standby but never responded to the base, Dillon said.

The planned drill never happened, he added.

The base tweeted that "reports of a real-world active shooter situation at the medical facility were miscommunicated before the exercise was able to begin."

"There was a misidentification of the security forces emergency services team who were conducting a routine inspection of the facility," the base said.

Vice President Joe Biden was expected to leave from the base today for Ohio.

The U.S. military facility, about 17 miles outside Washington, D.C., is home to Air Force One.