The US Capitol was briefly evacuated after police identified an aircraft that they said posed “a probable threat”, but which was actually carrying members of the US Army Golden Knights who parachuted into a baseball stadium for a pre-game demonstration.

The alert from the US Capitol Police sent congressional staffers fleeing from the Capitol and legislative building around 6.30pm local time.

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.

House Speaker Nancy Pelosi blamed the Federal Aviation Administration (FAA) in a statement, saying its “apparent failure to notify Capitol Police of the pre-planned flyover Nationals Stadium is outrageous and inexcusable.” The FAA did not respond to a request for comment.

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.

The US Army parachute team the Golden Knights descend into National Park
It did not appear any of the US capital’s defence systems were scrambled during the incident (Alex Brandon/AP)

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 co-ordinating its flight with the control tower at nearby Ronald Reagan Washington National Airport.

Investigators were still working to determine why the event was not 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 and military aircrews on constant alert, but it did not appear 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 authorised to discuss the matter publicly and spoke to the AP on condition of anonymity.