WASHINGTON (AP) — The U.S. Capitol was briefly evacuated Wednesday evening after police identified an aircraft that they said posed “a probable threat” — but the plane was actually carrying members of the U.S. Army Golden Knights, who then parachuted into Nationals Park for a pregame demonstration.

The alert from the U.S. Capitol Police sent congressional staffers fleeing from the Capitol and legislative building around 6:30 p.m.

The incident suggested a stunning communications failure between the military and the Capitol Police, all the more remarkable because of Washington’s focus on improving security since the January 6, 2021, attack on the building by supporters of then-President Donald Trump.

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Many who work on Capitol Hill have remained on edge more than a year after hundreds of pro-Trump rioters pushed their way past overwhelmed police officers, broke through windows and doors and ransacked the Capitol as Congress was voting to certify Joe Biden’s electoral win.

In Wednesday’s incident, the aircraft, a twin-engine plane, took off from Joint Base Andrews in Maryland and had been circling inside heavily restricted airspace close to the Capitol when the alert was sent. Radar tracking data shows the plane, a De Havilland Twin Otter, remained clear of the prohibited airspace over the Capitol Building and other government complexes at all times. Air traffic control recordings capture the army plane coordinating its flight with the control tower at nearby Ronald Reagan Washington National Airport.

Investigators were still working to determine why the event wasn’t properly coordinated with law enforcement officials in Washington, two people familiar with the matter told The Associated Press. Multiple federal agencies began scrambling officials as the plane circled overhead.

The capital region is defended by several surface-to-air missile sites, as well as military aircrews on round-the-clock alert. It did not appear that any of those systems were scrambled.

Officials believe, based on a preliminary review, the pilot may have not properly reported taking off or had appropriate clearance, the people said. They were not authorized to discuss the matter publicly and spoke to the AP on condition of anonymity.

The plane landed back at Andrews around 6:50 p.m. after the parachuters descended into the middle of the field at Nationals Park. The stadium, home of the Washington Nationals baseball team, is a little more than a mile away from the U.S. Capitol.