A ‘slow moving’ aircraft which was detected on radar over Washington D.C. sparking a lockdown at the White House and Capitol could actually have been a flock of birds.

Secret Service agents issued a lockdown order at the White House on Tuesday morning, citing a ‘shelter in place’ order for an ’emergency’ that lasted just over a half-hour.

A U.S. Capitol Police source said shortly before 9:00 a.m. that there was an ‘unauthorized aircraft’ near Washington, D.C.

CNBC reported that fighter jets were scrambled. Joint Base Andrews, the nearby Air Force facility that handles Air Force One traffic, did not have an immediate comment.

Capitol Police sources told CNN later that a ‘big slow-moving blob of something’ on radar sparked the alert, with fears it could be a small aircraft or drone.

Coast Guard helicopters which arrived to intercept the threat reported that they couldn’t see any aircraft in the restricted airspace.

Capitol Police sources told CNN the radar return could be a flock of birds, given its slow movement and how it later dispersed.

And air traffic control audio shows that a military pilot reported seeing a flock of birds as they responded to the emergency.

‘A trained radar operator should be able to differentiate between a flock of birds and an aircraft, but this mistake is actually a fairly common one’, retired Air Force Col. Cedric Leighton told CNN.

The alert sparked concern across Washington D.C. as both the White House and Capitol were placed on lockdown.

A reporter for The Independent reported seeing movement at a missile launch site near the U.S. Capitol, where similar lockdown orders were issued.

CNN reported on the air that the Pentagon had convened a ‘national event conference’ although the plane was not thought to be ‘hostile.’

National event conferences are interagency task forces put together at a moment’s notice to deal with fast-evolving national security or public health emergencies.

Reporters in the White House press briefing room were told to remain indoors as the drama unfolded, and the door was locked from the outside.

Construction workers building a replacement fence on the north side of the presidential mansion were evacuated shortly after 8:45 a.m.

‘They said, “Everybody get out.” Just like that,’ one hard-hatted worker told DailyMail.com.

‘It’s “shelter-in-place”,’ a uniformed agent patrolling on the west side said. Another confirmed that ‘there’s an emergency inside.’

‘We need you to shelter that way,’ he said, pointing across 17th street.

West Wing staffers with telltale blue badges were next to exit in smaller, straggling numbers. Two of them shrugged their shoulders at a question about what caused the evacuation.

‘Don’t know,’ one woman said. ‘They don’t tell us. They just said to go.’