Something of a mystery has emerged after a brief lockdown of the White House and U.S. Capitol due to an alleged violation of the heavily restricted airspace over Washington, D.C. this morning. Reports now indicate that a flock of birds, a weather balloon, or something else besides a plane or helicopter, appears to have triggered the alert, which sent fighter jets and helicopters scrambling into the air. Whatever the case, the incident underscores the very real challenges involved in protecting the skies above the nation's capital, and short-range air defense, in general, especially as novel threats, such as increasingly capable small unmanned aircraft, continue to emerge and evolve. Some of the first reports of a potential aerial threat emerged just before 8:30 AM local time in Washington, with the U.S. Capitol Police and U.S. Secret Service subsequently restricting access to various sites and telling individuals at the White House and Capitol Building to shelter in place. Within 45 minutes, the U.S. Capitol Police said that the situation had passed "without incident" and the lockdowns ended shortly thereafter.

Before the lockdowns ended, the U.S.-Canadian North American Aerospace Defense Command (NORAD), responsible for coordinating the defense of airspace in both countries, including over Washington, D.C., Tweeted out that "We have tasked aircraft to respond. More details to follow."

"Senior officials across the interagency are monitoring the situation on a national event conference call. NORAD aircraft are on site and responding. Plane is not considered hostile at this time," the command said in a second Tweet that it subsequently deleted for unspecified reasons. U.S. Northern Command posted a nearly identical version of this statement on Twitter, which is still available. Pentagon spokesperson Lieutenant Colonel Chris Mitchell also told CNN virtually the same thing at the time.

At the time of writing, neither the Pentagon, U.S. Northern Command, nor NORAD has issued an official press release regarding the incident. "NORAD responded to an event in the Special Flight Rules Area surrounding Washington D.C. this morning. NORAD directed @USCG rotary wing aircraft to investigate and the event was resolved without incident," NORAD said in another Tweet at 12:50 PM, local time.

The U.S. Coast Guard rotary-wing asset was almost certainly that service's MH-65 Dolphin helicopter that is based at Coast Guard Station Washington, D.C., which is collocated with the U.S. military's Joint Base Anacostia-Bolling. This helicopter also operates at times from a separate site at Reagan National Airport. The MH-65, among its missions in the area, is on call to specifically to intercept low-and-slow moving targets. Fox News also reported that unspecified fighter jets and a U.S. Capitol Police helicopter responded to the incident. It's not immediately clear if the Capitol Police has a helicopter and this may have been the Coast Guard's MH-65. The fighter jets in question were almost certainly the F-16C Vipers from the D.C. Air National Guard that sit on alert at Andrews Air Force Base in order to respond to just these kinds of situations. Ground-based air defense systems that also help protect the capital were also no doubt on alert during the incident. Sara Cook, a CBS News producer covering the White House, also Tweeted out a picture showing a glimpse of an AN/TWQ-1 Avenger point air defense system that is positioned on top of a building within line-of-sight of the White House. The AN/TWQ-1 is more typically seen mounted on a Humvee, but can be positioned in fixed emplacements, as well. It is armed with the two four-round launchers for the FIM-92 Stinger missile and a .50 caliber M3P machine gun. National Advanced Surface-to-Air Missile Systems, or NASAMS, which include ground-based launchers that fire the AIM-120 Advanced Medium-Range Air-to-Air Missile (AMRAAM), are also part of this greater integrated air defense network around Washington, D.C.

As for the actual target that these assets were search the sky for, at no point during the incident did any U.S. government agency identify the threat beyond the statements that it was a "plane," which NORAD, at least, has now deleted. Sam Sweeney, a D.C. Morning and transportation reporter at ABC7 television in Washington, subsequently Tweeted out, citing unnamed sources, that a "flock of birds appearing on radar" had been the real culprit.