JOINT BASE ANDREWS, Md. (AP) — A military post near Washington said a lockdown was lifted Thursday except for a medical building where an active shooter was reported earlier in the day.

Joint Base Andrews said in a tweet about 10:20 a.m. that the all-clear was given for the base except for the medical building. The base did not say why the Malcolm Grow Medical Facility remained on lockdown.

A law enforcement official says no active shooter was found at the military post outside Washington.

The law enforcement official was not authorized to discuss the matter by name and spoke on condition of anonymity.

The base was placed on lockdown about 9 a.m. after an active shooter was reported. The base is home to Air Force One and is about 20 miles from Washington.

Even after the lockdown, it was not immediately clear if any shots had been fired.

Vice President Joe Biden was scheduled to leave from Andrews on Thursday morning, but his trip was delayed by the lockdown.

The vice president's office said he was waiting out the delay at his residence at the Naval Observatory in Washington.

Biden was due in Columbus, Ohio, for a midday campaign event for Gov. Ted Strickland.

The president, vice president and other senior government officials fly in and out of Joint Base Andrews.

President Barack Obama was last at the base Wednesday night when he returned from a trip to Ottawa, Canada

Earlier Thursday, Homeland Security Secretary Jeh Johnson said there was an "unfolding situation" at the base but provided no other details.

Emergency vehicles in the area of the base had on lights, but no sirens. At least three people in camouflage and helmets could be seen walking working dogs around the three-story building. About 10:15 a.m., a few people could be seen walking out of the building, including a person being moved in a wheelchair.

Helicopters hovered overhead, but it was unclear if they belonged to news organizations or the military.

Rodney Smith, the patient advocate at the Andrews medical facility, said an active-shooter exercise was scheduled for Thursday morning, and then he was told it was a "real-world" situation. He said he was on lockdown and didn't have any more information.

"First it was an active-shooter exercise. Then it came back 'real world,'" Smith said by phone Thursday morning.

Smith said the situation was unfolding at the newer of two buildings at the Malcolm Grow Medical Facility. He was in the older building.

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Tucker reported from Washington. Associated Press Writers Sarah Brumfield and Ben Nuckols in Washington contributed to this report.